Conjugate Heat Transfer Characteristics Research Articles

Computational fluid dynamics of flow boiling and conjugate heat transfer characteristics in a mini/micro-channel printed circuit steam generator

Numerical investigation of printed circuit heat exchanger PCHE was carried out for the water-steam system to optimize the design parameters for the possible use of PCHE as a steam generator in integral nuclear reactors- thus targeting efficient renewable energy by enhancing heat transfer. Heat fluxes, overall heat transfer coefficient (OHTC), and volume fraction of steam in a single channel of PCHE are reported for counter-current flow arrangement. The numerical model consisted of 1mm∗1.5mm single hot and cold channel. The heat transfer parameters were calculated using the Volume of Fluid (VOF) model in ANSYS Fluent with boundary conditions: top/bottom adiabatic; left/right periodic; mass flow inlet and pressure outlet. The effect of cold side degree of inlet subcooled (DISC) – varied from 100 to 0 K with a decrement of 20 K- and mass flux (200 to 500 kg/m2.s on OHTC, vapor fraction, pressure drop, and effectiveness was studied. For the fixed degree of subcooled, the OHTC and pressure drop increase as cold side mass flux increases. Similarly, for the fixed mass flux at varying DISC, the OHTC increases and pressure drop decreases for low mass fluxes up to 200 kg/m2.s because most of the channel remains in the nucleate boiling regime. However, with further increase in mass flux opposite trends in OHTC and pressure drop were observed due to single phased regime. The vapor fraction of steam increases with lower DISC at low mass fluxes and decreases with the increase in mass flux. The effectiveness decreases as the cold side inlet temperature approaches saturation temperature at 3 MPa.

Study on coupled heat transfer model and enhanced heat transfer law of liquid metals and supercritical fluids

ABSTRACT The supercritical carbon dioxide Brayton cycle system is being considered for application in the power conversion system of advanced liquid metal fast reactors. One important research topic is the coupled heat transfer characteristics and enhanced heat transfer laws between liquid metals and supercritical fluids. However, the traditional Reynolds analogy hypothesis with a constant turbulent Prandtl number is difficult to satisfy the numerical conjugate heat transfer research of liquid metals and supercritical fluids. Therefore, based on the open-source computational fluid dynamics program OpenFOAM, a numerical method suitable for the coupled heat transfer calculation of liquid metals and supercritical fluids was developed, which can achieve precise conjugate heat transfer solutions by applying different turbulence models and heat flux models to liquid metals and supercritical fluids. Subsequently, based on this method, the coupled heat transfer behavior of liquid metals and supercritical fluids in the straight channel of a typically printed circuit heat exchanger was studied. The focus was investigating the conjugate heat transfer characteristics of liquid heavy metal lead-bismuth eutectic and liquid metal sodium with supercritical carbon dioxide. Some sensitive results, including the inlet temperature and Reynolds number of liquid metals and supercritical fluids, were qualitatively and quantitatively analyzed.

Conjugate heat transfer characteristics numerical simulation on steam generator tube of lead-cooled fast reactor

The safety assessment of the Steam Generator Tube Rupture (SGTR) accident is a crucial step in the design process of the Lead-cooled Fast Reactor (LFR). To begin with, it is necessary to simulate the steady-state heat transfer of the LFR Heat Exchanger (HX) in order to determine the initial conditions of the SGTR accident. In this study, the heat transfer characteristics of the fluids on both sides of the LFR HX are analyzed. The appropriate models are modified based on the heat transfer characteristics of liquid metal and boiling water. The accuracy of the boiling models is verified by comparing the simulation results with the outcomes of boiling heat transfer experiments. Additionally, the simulation results are compared with the results obtained from ATHLET calculation and empirical formulas. On the secondary side, the supercooled water undergoes heating until it reaches the superheated steam phase. The preheating section spans from 0 to 600 mm, the saturated boiling section ranges from 600 to 3000 mm, and the superheating section extends from 3000 to 6600 mm. Ultimately, the distributions of steady-state temperature, pressure, and vapor volume fractions of the LFR HX are determined, providing the necessary initial conditions for the occurrence of the SGTR accident.

A study of pore scale flow and conjugate heat transfer characteristics in real and Kelvin anisotropic foams

Porous foam is a promising material in the field of science and engineering owing to its potential to offer heat transfer enhancement. In this work, an ideal Kelvin structure with considering anisotropy is constructed based on the characteristic parameters extracted from a real foam, and the flow and heat transfer characteristics in real and Kelvin foams is numerically investigated and compared by an in-house MRT-TLBM solver accelerated by a GPU. Results demonstrate that when the flow is along the elongation direction of the structure, the permeability is larger and Nusselt number is lower than those in the other directions for both real and Kelvin foams. Compared to the real foam, the Kelvin foam is more prone to inducing turbulence. For the overall performance of flow and heat transfer, the real foam behaves better at small Reynolds number (Rec <750), while Kelvin foam does when Rec exceeds 750 due to the different dominant mechanisms of heat transfer. In particular, the comprehensive factor (θ) of Kelvin foam is 12.8 % larger than that of the real foam at Rec=2000.

Comprehensive evaluation on conjugate heat transfer and thermal stress performance of film cooling

Film cooling is a commonly used cooling technology in gas turbines, which could reduce heat load by spreading cooling air. However, it faces the problem of stress concentration in structures with holes. The present work focuses on conjugate heat transfer and thermal stress characteristics for a flat film cooling plate through an experimentally validated numerical approach. It discusses the effect of three geometry factors: inclination angle, compound angle, and ellipticity. An engine case simulation and a laboratory case simulation are compared to validate the analogy principle of conjugate heat transfer and thermal stress analysis derived from theoretical analysis. According to the analogy principle, the laboratory flat plate film cooling cases are set to conduct numerical studies. The maximum Mises stress of the film hole is equal to the product of the reference stress and the stress concentration factor. The reference Mises stress is proportional to the overall cooling effectiveness. The stress concentration factor is mainly related to the geometry of the film hole. Reducing the inclination angle of the film hole from 90° to 30° increases the overall cooling effectiveness and drastically increases the stress concentration factor from 2.7 to 5.5. Based on a cylinder hole with a 30° inclination angle, increasing the compound angle enhances the overall cooling effectiveness and slightly increases the stress concentration factor. Based on a cylinder hole with a 30° inclination angle, increasing the ellipticity increases the overall cooling effectiveness and reduces the stress concentration factor. With the increase of ellipticity from 0 to 4, the stress concentration factor decreases from 5.5 to 2. Elliptical film holes are ideal for future low-stress, high-cooling-efficiency turbine blades to extend the life of cooling vanes.

Conjugate Heat Transfer Characteristics of a Film-Cooled Turbine Blade Leading Edge With Staggered-Oblique Impinging Jets

Abstract The turbine blade leading edge is subjected to harsh conditions due to high heat loads and unfavorable compact structures. To improve the cooling performance, a novel impingement scheme is proposed to be used in a film-cooled turbine blade leading edge. Different from a normal impingement scheme, the novel scheme consists of two rows of staggered impinging jets at oblique angles of ±35 deg, and is thus named as the staggered-oblique impingement scheme. A conjugated numerical investigation is carried out to illustrate the underlying mechanisms of the cooling performance. Three typical jet Reynolds numbers, 6000, 12,000, and 18,000, are studied using the validated SST k–ω turbulence model. Numerical results show a flow separation within the staggered-oblique impinging jets, which causes the discharge coefficient for the novel impingement scheme lower than that for the normal impingement scheme. Results also reveal a phenomenon difference that two symmetric vortices are induced by each normal impinging jet, while only one vortex appears on the acute angle side along with each staggered-oblique impinging jet. The flow fields of the staggered-oblique impingement scheme create a more uniform heat transfer distribution and a maximum of 23.7% higher area-averaged Nusselt number than the normal impingement scheme. The area-averaged overall cooling effectiveness for the staggered-oblique impingement scheme is higher than the normal impingement scheme by a maximum of 4.8%. The uniformity and the enhancement of the overall cooling effectiveness arise from the wall jet being fully developed and affecting a larger area of the target wall. The adiabatic cooling effectiveness is similar for both impingement schemes. This indicates that the improvement in overall cooling effectiveness for the staggered-oblique impingement scheme mainly arises from internal heat transfer.

Effect of non-uniform heating on conjugate heat transfer and entropy generation characteristics within a partially filled porous corrugated channel

The persistence of the present numerical investigation is to evaluate the thermal and total entropy generation characteristics of flow within a partially filled porous wavy channel with finite wall thickness subjected to non-uniform wall heat flux. The average Nusselt number and the total entropy generation increase with the permeability value of the porous medium (Da) irrespective of asymmetric wall heat flux amplitude and wavelength as well as with the variation in thickness ratio and thermal conductivity value of the solid substrate. However, the total irreversibility decreases for a high Darcy number value when the phase angle between the applied heat fluxes is varied. Moreover, the average Nusselt number decreases up to a critical value of phase angle followed by a gradual increment for Da = 10−5 and 10−3, respectively. On the contrary, the reverse trend can be seen for Da = 10−1. The optimal choice of nonuniform heat flux amplitude, wavelength, and phase angle for a high heat transfer rate with minimum irreversibility generation is found to be 10 W/cm2, 5 mm, and 180°, respectively for a thickness ratio of 0.5 and thermal conductivity value of 646.07 W/m·K independent of Darcy number.

An experimental and numerical study on the blade tip conjugate heat transfer performance

In the gas turbine, the blade tip is under extreme heat load and thereby it is necessary to adopt cooling method for the tip region. In this work, a blade tip conjugate heat transfer model was conducted in a linear cascade. The tip overall cooling effectiveness (φ) and adiabatic cooling effectiveness (η) were measured by the pressure sensitive paint (PSP) and the Infrared thermography (IR), respectively. Impacts of coolant blowing ratios (M = 1.0 ∼ 2.5), tip gap size (τ = 1.98%, 3.97%C, chord length), and film holes locations on the tip φ distributions were investigated. Then, the numerical simulations were performed to obtain the tip solid temperature distributions, and to reveal the conjugate heat transfer characteristics. Within the scope of this work, both the tip φ distributions and the tip solid temperature distributions indicate that the convection in film holes is a major factor of the tip conjugate heat transfer process. The tip area-averaged φ for the CL_H case are increased evidently by 13.1%∼38.9% with the increased coolant M, whereas the increase of η is not obvious. The tip gap size has less effect on the tip φ distributions than the coolant M, especially at a low M. The higher φ and the lower tip solid temperature region are detected near the film holes for any film hole location. The area-averaged φ values of the cases with film holes along the chamber line are 10.6% and 3.0% higher than those of the cases with film holes along the pressure-side or suction-side rim, respectively. This work revealed the major factor in the tip conjugate heat transfer process and may serve as a guide for tip cooling design.

Assessment of flow and heat transfer of triply periodic minimal surface based heat exchangers

An efficient intermediate heat exchanger is crucial for ensuring the safe operation, lifespan and energy efficiency of the advanced nuclear systems. Benefiting from the development of additive manufacturing technology, it is possible to design and optimize heat exchangers by introducing triply periodic minimal surface (TPMS) structures. With the excellent mechanical and thermal performance of TPMS, further improvements in the energy efficiency of advanced nuclear systems are achievable. In this study, I-WP surface, Primitive surface based heat exchangers and a printed circuit heat exchanger for accelerator driven subcritical systems are constructed in three dimensions. In order to obtain the potential enhancement of thermal performance of TPMS-based heat exchangers, the fluid flow and conjugate heat transfer characteristics in TPMS heat exchangers, especially for the specific working medium of lead-bismuth eutectic (LBE) are investigated. A parametric analysis is conducted on the key design variables including the solid volume fraction and hydraulic diameter. The results indicate that TPMS-based heat exchangers could achieve about 2–3 times the total heat transfer rate with half the volume of a printed circuit heat exchanger. The TPMS topologies contribute greater heat transfer enhancement on the Helium side than the LBE side, which helps to optimize the thermal resistance allocation in Helium-LBE heat exchangers. An increase of solid volume fraction enhances the convective heat transfer of Helium in I-WP heat exchanger, while homogenizing the local thermal performance. This study provides a database for the design and optimization of TPMS-based heat exchangers, which contributes to the sustainable future target.

The effect of coupled combustion reaction on conjugate heat transfer in boundary layer of a segregated oxidizer/fuel solid motor

Combustion reaction plays an important role in heat transfer of boundary layer of solid rocket motor. In the present study, a transient numerical model is developed to investigate the influence of coupled combustion reaction on heat transfer in boundary layer of the segregated oxidizer/fuel solid motor (SOFSM). The results show that conjugate heat transfer characteristics of non-reacting flow are distinct from those of the reacting flow with coupled combustion. The reacting flow with coupled combustion renders higher and more homogeneous temperature and velocity compared with the non-reacting flow. Due to the effect of combustion heat release on thermophysical properties of the gas phase species, the heat transfer coefficient and effective thermal conductivity of the reacting flow are much higher than those of the non-reacting flow. When the combustion time increases, the heat transfer coefficients decrease in the reacting and non-reacting flows due to reduced flow velocity and lower O2 concentration. The Nusselt number (Nu) of the reacting flow is also higher than that of the non-reacting flow. Significant decrease of Nu is found in the non-reacting flow with combustion time. However, the Nu varies very slightly along the flow direction in the reacting flow. As pressure increases from 1.5 MPa to 5.0 MPa, the high-temperature area becomes narrower and the flow velocity decreases in the combustion channel. The diffusion and mixing of the oxidizer and fuel gas along the radial direction are thus weakened in the coupled combustion flow. The total heat transfer coefficient decreases from 295.88 W m−2 K−1 to 46.52 W m−2 K−1 and Nu decreases from 36.27 to 16.56 with increase of pressure. The distribution of total heat transfer coefficient is found to be highly synchronous to the distribution of regression rate of coupled combustion process. The distribution of heat transfer coefficient directly affects parallel regression and coupled combustion of the TAGN propellant. The fitting relationship of Nu, i.e., Nu = 0.058Re0.6Pr0.424 is obtained for the coupled combustion flow in the motor, which provides better insight into the heat transfer in the SOFSM system.

Numerical investigation on conjugate heat transfer characteristics of film and vortex composite cooling under rotating conditions

This paper numerically investigates the vortex and film composite cooling performance under rotating conditions. The cooling performance of the adiabatic and conjugate models is compared under the range of 0–4000 r/min. The conjugate model contains the fluid region: the cascade, the film holes, and the internal vortex cooling chamber, as well as the solid region: the blade material between the internal flow and the mainstream flow. The adiabatic model is established by removing the blade material part in the conjugate model. The dimensionless temperature θ inversely proportional to the temperature is adopted. Results show that the blade leading edge temperature doesn’t vary linearly with the rotating speed. The stagnation line of the mainstream flow on the blade leading edge moves from the pressure surface to the suction surface. The maximum θ appears at 1500 r/min when the stagnation line stays certainly on the row of film holes located on the pressure surface and is 7.86% higher than the minimum θ. The minimum θ appears at 2500 r/min when the stagnation line stays on the position between the rows of film holes. The distribution of θ is much uniform, and the value of θ is much higher in the conjugate cases than the adiabatic cases due to the heat conduction through the blade material. The highest aerodynamic parameter appears at 2000 r/min due to its relatively low blade leading edge temperature and low coolant consumption and is 41% higher than the aerodynamic parameter at 0 r/min.

Coupling heat transfer study of liquid sodium and supercritical carbon dioxide in a PCHE straight channel based on a four-equation model

The Brayton cycle using supercritical carbon dioxide (S-CO2) as the working fluid is expected to be a thermo-power conversion technology for sodium-cooled fast reactors. Printed Circuit Heat Exchanger (PCHE) is the key heat transport structure to realize the above technology. The study of conjugate heat transfer characteristics of sodium (Na) with low Prandtl number turbulent heat transfer characteristics and S-CO2 with supercritical convective heat transfer behaviors in a PCHE channel are of great significance. However, the constant turbulent Prandtl number (Prt) model obtained by the general Reynolds analogy hypothesis may affect the conjugate heat transfer assessment between Na and S-CO2. Compared with the constant Prt model, the fluid’s Prt distribution can be obtained by four-equation models, introducing the transport of turbulent kinetic energy k, dissipation rate ε of k, temperature fluctuation kθ, and dissipation rate εθ of kθ. The four-equation model is expected to obtain more effective conjugate heat transfer characteristics between Na andS-CO2, while there are no available commercial codes. Therefore, in this paper, two kinds of four-equation model, which can effectively evaluate the heat transport performance of Na with low-Prandtl number and S-CO2 away from the pseudo-critical region, respectively, are first introduced into the conjugate heat transfer solver of OpenFOAM. Then the conjugate heat transfer experiment data of Na in a pipe and the simulation data of S-CO2 in a PCHE straight channel are used to verify the validity of the numerical model in this paper. The results show that the present calculation method can effectively reproduce the numerical conjugate heat transfer process of Na and S-CO2. Finally, based on four-equation models, the coupling heat exchange performance of Na/S-CO2 are studied to obtain the effects of mass rate and temperature on the conjugate heat transfer process between Na and S-CO2 in a PCHE straight channel.

Study on the micro conjugate heat transfer characteristics of coal reconstructed from CT images under the influence of temperature

Heat injection in coal seams, a new method to enhance gas drainage, is faced with problems such as low heat conduction efficiency and considerable heat dissipation. Based on experiments, this study reconstructed a micro heat transfer model through CT images to discuss the interaction between coal temperature rise-induced fracturing and the heat transfer performance of coal, which can provide a new insight into efficient heat injection and mining in coal seams. First, coal samples were modified by being heated at four temperatures and then scanned in situ by an industrial micro CT. Then, fractures were reconstructed by the interactive threshold segmentation and interactive top hat method to generate a 3D mesh model. Meanwhile, a conjugate heat transfer model including fracture fluid and coal matrix was established. Finally, the simulation result of thermal conductivity was compared with the results of the hot wire method and the laser flash method, and the influence of thermal fracturing structure evolution on coal heat transfer was analyzed. The following findings were obtained: The conjugate heat transfer model is applicable to fractured coal. The average relative error of its thermal conductivity is 5.85%, and it boasts high accuracy in characterizing heterogeneity and anisotropy. The fracture propagation and water evaporation of coal block heat transfer, while the adhesion and crystal contact surface expansion of heated coal are conducive to heat transfer. These two mechanisms compete, ultimately affecting the thermal conductivity. After the coal sample is heated within the temperature range of 40–100 °C, the fracture damage coalescence enhances the anisotropy of heat transfer in coal. The maximum difference in thermal conductivity between the bedding direction and the direction perpendicular to bedding is 0.181 W/(m·K), about 4.6 times larger than the initial difference. The research results lay a theoretical foundation for improving the heat transfer efficiency of heat injection in coal seams.

Study on conjugate heat transfer characteristics of flexible structure of solid motor nozzle based on multiple working conditions

In order to study the influence of the deformations of the nozzle’s flexible sections on its inner wall’s heat transfer, this research uses the two-dimensional axisymmetric model and the k − ω SST turbulence model to numerically analyze the tail flow field of the flexible-extendible nozzle under multiple working conditions. The flow field and the inner wall’s heat transfer of the flexible sections with and without deformations are simulated and analyzed. The simulation results show that when the flexible sections are deformed, new shock waves will be generated in the tail flow field of the nozzle; the temperature and the heat flux of the first half of each flexible section decrease, while those at the end of each flexible section increase, and the deformations’ effect on temperature and heat flux diminishes over time. The research shows that the deformations of the flexible sections will make the inner wall’s heat transfer of the flexible sections uneven, and the inner wall’s ablation at the end of each flexible section may be aggravated, while that of the front half may be alleviated.

Experimental decoupled-analysis of overall cooling effectiveness for a turbine endwall with internal and external cooling configurations

High-efficient composite cooling structures that are comprised of external and internal cooling techniques, are one of the key technologies to guarantee the aerothermal performance and reliability of a turbine vane, especially the endwall region due to unique complex flow structures. It is thereby critical to understand the flow mechanism and conjugate heat transfer characteristics in this region for further optimal design. In this paper, a representative endwall model which matches non-dimensional parameters to real turbine operating conditions is investigated. Thermal protection for the endwall is provided by internal jet impingement and external purge flow and discrete hole injection cooling. Comprehensive measurements of overall cooling effectiveness over the endwall, total pressure loss coefficient, and non-dimensional temperature at the cascade exit are performed by using an infrared (IR) thermography technique, a five-hole probe, and a temperature probe. Through decoupled analysis, heat transfer and flow characteristics of film cooling only, both jet impingement and film cooling, upstream purge cooling only, and combined cooling are analyzed, respectively, and their optimal ranges of cooling air are obtained. The measurements reveal that with film cooling only, only the regions around the film holes are covered. Using both film and jet impingement cooling, cooling coverage is expanded, and cooling effectiveness is much higher with a more uniform distribution pattern. Purge flow achieves thermal protection for upstream regions of endwalls, and the combined cooling scheme that involves all cooling sources provides the best cooling performance. Globally, those cooling schemes have no obvious effects on overall cascade aerodynamic performance but affect the development and distribution of secondary flows.

Conjugate heat and mass transfer at heterogeneous burning surface of AP/HTPB/Al composite propellant at 2–10 MPa

Conjugate heat and mass transfer at heterogeneous burning surface has significant effect on combustion characteristics of AP/HTPB/Al composite propellant. In the present study, the conjugate heat and mass transfer characteristics of AP/HTPB/Al composite propellant are studied experimentally and numerically based on formulated combustion model and experimental facilities. With the increase of pressure and Al content, flame temperature increases significantly and leads to stronger heat transfer to the burning surface. With the content of Al varying from 18 wt% to 9 wt%, the total heat transfer coefficient decreases for all the sampled positions of the burning surface with a maximum decrease of 12% at the AP/HTPB interface. It is found that heat convection accounts for more than 97% of the total heat transfer coefficient. However, with pressure increasing from 2 MPa to 10 MPa, the total heat transfer coefficient and the respective contribution of heat convection and radiation vary slightly. Owing to the heterogeneous sandwich-like structure of the propellant, the temperature distribution at the burning surface of propellant is highly non-uniform. Therefore, heat transfer characteristics vary significantly at different positions of the burning surface. The heterogeneity of the burning surface is exacerbated by the non-uniform distribution of heat transfer coefficients at different positions of the burning surface. The burning rate of the propellant varies from 0.5 mm/s to 9.5 mm/s for different positions of the burning surface at 10 MPa. The results of micro-scale flame structure show that the mass transfer of fine Al particles in the combustion of AP/HTPB/Al composite propellant can be basically divided into four stages, i.e., migration, melting, agglomeration and oxidation/combustion. Coral reef-like aggregate structure is formed by some of the fine Al particles before oxidation and combustion. Increasing pressure is conducive to avoiding formation of large Al agglomerates and enhancing mass transfer at the heterogeneous burning surface.

Numerical study on conjugate heat transfer characteristics of liquid lead-bismuth eutectic and supercritical carbon dioxide in a PCHE straight channel

The Printed Circuit Heat Exchanger (PCHE) is the optimal heat transfer structure of the fourth-generation advanced Lead–Bismuth Eutectic (LBE) cooled fast reactor coupling with the supercritical carbon dioxide (S-CO2) Brayton cycle system. It is of great significance to study the conjugate heat transfer characteristics between LBE with low Prandtl number turbulent heat transfer characteristics and S-CO2 with supercritical convective heat transfer characteristics. However, the conventional Reynolds analogy assumption will affect the numerical heat transfer precision of LBE and, thus, the accuracy of the coupled heat transfer of LBE and S-CO2. In the present work, an advanced two-equation turbulent heat transfer model, which can effectively correct the numerical heat transfer process of LBE, is introduced into the conjugate heat transfer solver of the open-source Computational Fluid Dynamics (CFD) program OpenFOAM, where the Reynolds analogy assumption is retained to reproduced the heat transfer of S-CO2 flow away from the pseudo-critical region. The currently developed model and numerical method are compared with the experimental data of an LBE-cooled pipe and the simulation data of conjugated heat transfer of S-CO2 and S-CO2 in a PCHE straight channel. The results show that the calculation method in this study can improve the numerical conjugate heat transfer accuracy of LBE compared with the Reynolds analogy assumption model and can reproduce the flow and heat transfer process of S-CO2 in a PCHE straight channel. Then, the conjugate heat transfer characteristics between LBE and S-CO2 in a PCHE straight channel are studied. The effects of inlet Reynolds number and inlet temperature are focused on to study the conjugate heat transfer law of LBE coupling S-CO2.

Conjugate heat transfer and flow analysis on double-wall cooling with impingement induced swirling and film cooling

A novel double-wall cooling configuration with impingement induced swirling film cooling is investigated in present paper. The effects of impingement inclined angle, blowing ratio, film hole length-diameter ratio and Biot number on conjugate heat transfer characteristics are studied by numerical simulations. The formation, development and evolution of the vortex and the reconstruction process of vortex structures are analyzed. The results indicate that impingement inclined angle can increase the swirling intensity and destroy the kidney-shaped vortex of external film cooling. The spanwise average overall cooling effectiveness is improved by about 20% when the impingement inclined angle is increased from 15° to 30°. Increasing blowing ratio makes the kidney-shaped vortex gradually evolve into multiple vortex structures in the mainstream, thereby increasing the adhesion of coolant and improving the overall cooling effectiveness. The overall cooling effectiveness fluctuates with the increase of length-diameter ratio, due to the large length-diameter ratio destroying the external film cooling. When the Biot number is small, the internal impingement induced swirling cooling determines the cooling performance, and when the Biot number is large, the cooling performance is mainly affected by the external film cooling.

Unsteady conjugate heat transfer characteristics in hexagonal cavity equipped with a multi-blade dynamic modulator

PurposeThe transient analysis of the thermal response and frictional loss characteristics for flow-modulated conjugate heat transfer phenomena has been investigated in the present study. The flow domain is a partitioned cavity of a hexagonal structure equipped with a multi-blade flow modulator. The clockwise rotating blade is adiabatic and stirrers the internal flow along with the natural convection caused by the bottom heated floor of uniform heat flux. The conjugate behavior is introduced through the solid subdomains consisting of two brick-made partitions and one glass partition of uniform thickness. The material of the partition wall reflects the physical aspects of industrial applications. ApproachThe two-dimensional unsteady continuity, momentum, and energy equations are expressed in a non-dimensional form where the buoyant force is modeled through the Boussinesq approximation. The Arbitrary Lagrangian Euler (ALE) finite element is adopted to solve the moving mesh problem by formulating a free triangular discretization scheme. Parametric computational investigations are carried out for air as the working fluid (Pr = 0.71) and 3 different configurations of the rotating modulator while varying the other parameters, i.e., Reynolds number (Re) and Rayleigh number (Ra) for a fixed Biot number (Bi = 104). This dynamic mesh problem encompasses a wide range of parameters, i.e., (100 ≤Re≤ 103), and (104≤Ra≤ 106) for Bi = 104 to evaluate the thermodynamic response of the present thermo-fluid system. Various thermo-fluid system responses are visualized through the spatially average Nusselt number evaluated on the heated surface, system effectiveness, average thermal storage capacity, and frictional power loss of the flow domain. The thermal response is fragmented into individual responses in terms of component signal frequency by the Fast Fourier Transform (FFT) analysis. FindingsAccording to the current analysis, increasing the number of blades increases the rate of heat transfer by a negligible amount. On the other hand, among three different types of modulators, the three-blade modulator shows the lowest power loss coefficient and the lowest power instability. Thus, a three-blade modulator qualifies as the optimum design to impart external motion to the working fluid to introduce forced convection. LimitationsThis research has some scope for improvement. Future studies can be conducted on the improvement of blade designs, thickness, and size of the blades. Practical ImplicationsThe present study can shed light on the modeling of a better storage system where heat transfer and moisture control are integrated for the proper functioning of the storage, such as food storage, cold storage, garment inventory, etc. OriginalityThis study is original, and no previous research has been conducted considering the hexagonal domain and variation of the number of blades. ConclusionThe present study reveals that a modulator best serves its purpose at low Rayleigh and high Reynolds numbers. Moreover, the three-blade modulator has the lowest power loss coefficient and lowest power instability, making it the most suitable modulator to impart external excitation to the working fluid.

Heat exchange structure design and overall performance analysis of double-concentrator solar thermal thruster

In this paper, the platelet heat exchanger in the thermal propulsion system of micro satellite with liquefied gas as propellant was designed, the complete gasification and stabilization scheme of propellant was proposed. The unsteady conjugate heat transfer (CHT) characteristics between the heat exchanger and gaseous propellant were analyzed, and the cumulative data of error estimation in the numerical simulation process was provided to ensure the credibility of the results. The CHT computational fluid dynamics (CFD) simulation of the platelet 3D model under steady-state conditions was carried out to find the empirical correlation between the average Nusselt number and the average Reynolds number. A simplified unsteady 1D model of heat exchanger was established by using the loose coupling method based on quasi steady flow field. The results show that the platelet structure could heat the working medium to more than 2379 K with the heat transfer efficiency of 86%, the outlet temperature and heat transfer efficiency of the heat exchanger were maintained at 1871 K and 69% in steady state when the mass flow rate was 1.14e-6 kg/s. The 1D model could accurately reflect the real heat transfer situation to a certain extent, the simulation error was less than 8.5% compared with the 3D model, and the calculation time was greatly shortened, making the adjustment of heat transfer strategy more convenient. Intermittent heat transfer method could keep the outlet gas temperature, thrust and specific impulse of the thruster at a high level to improve the performance of the system. Taking the nanosatellite with a total mass of 7.55 kg and a propellant mass of 0.25 kg as an example, when the duration of valve open time was 1.5s, the velocity increment of the satellite reached 72.21 m/s with the specific impulse of over 2400 m/s, which was twice than that of nanosatellites without heat exchange structure.