Pure Heat Transfer Research Articles

Thermal performance of a novel hybrid heat pipe with composite heat transfer characteristics

Considering the capillary threshold of oscillating heat pipes (OHP), a novel hybrid heat pipe (HHP) was designed with channels with hydraulic diameters that alternately exceed and fall below this threshold, parallelly arrayed within a unified copper substrate. This technology attains a comprehensive heat transfer performance within the HHP, seamlessly transitioning from pure phase change heat transfer to hybrid pulsating flow heat transfer to effectively accommodate the escalating demands of heat load. Performance and visualization are used to analyze the operational characteristics of HHP. The results indicate that the HHP mainly transfers heat through the phase change in the large channels in lower heat input, while the working fluid in the small channels remains calm. Once the heat input is sufficient, the geyser boiling in the large channels and the liquid slug pulsating in the small channels jointly carry out heat transfer. Furthermore, the liquid content in the large and small channels can be adaptively adjusted to each other. The high-speed liquid in the small channels can be squeezed into the large channels to ensure the sufficient pulsating space and achieve efficient heat transfer under high heat input. In addition, the operation of the HHP can be significantly impacted by the filling ratio (FR). In the phase change stage, the FR of 47.6 % can be considered as the turning point. After transitioning to the hybrid pulsating flow stage, the HHP with low FR can get a better performance. This new type of heat pipe with gradually changing operation characteristics may be critical to solve thermal problems in variable heat load domain.

Investigation of thermal performance at forced convection in plate-fin heat sink

Interest in studies on more effective and faster heat dissipation in microelectronic equipment has increased in recent years due to the cooling area and equipment size limitations. This study attempts to focus on the effect of different fin geometry parameters on pure forced convection heat transfer. Total surface areas of plate-fin heat sinks were kept constant and were carried out for ten different heat sinks in these analyses. The Ansys-Fluent 2023 R1 commercial package program was used to perform the thermal and hydrodynamic performance analyses. Three-dimensional turbulent forced convection heat transfer analyses at heat sinks were subjected to fluid flow (500≤Re≤3000). The effects of gap between fins, fin thickness, fin height, fin number, and air velocity on the heat transfer and the friction factor were obtained and discussed for heat sinks. It was observed that there was a maximum difference of 0.27 % between the maximum temperatures obtained as a result of this analysis and the experimental study results in the literature. Among the ten cases examined (from Case 1 to Case 10), the highest heat transfer coefficient was obtained in Case 2 (number of fins 6, gap between fins 4 mm, fin height 17 mm and fin thickness 2 mm) at Re = 3000. The heat transfer coefficient of Case 2 was 41.29 % higher than the heat transfer coefficient of Case 4 at Re = 3000; though, the friction factor of Case 2 is 55.93 % higher than Case 4. Also, a useful correlation for the heat transfer coefficient was generated for the fin geometric parameters and Reynolds number.

Study on temperature distribution of vertical liquid falling film heat transfer with Al2O3 nanofluid

This study presents a laminar smooth 2-D theoretical model of the falling liquid film using an in-house code and validated with experimental results. The numerical model considers thermophysical property enhancements with nanoparticles Al2O3 to different liquids such as water, ethylene glycol (EG), and LiBr-H2O. An experimental setup has been developed to validate the pure heat transfer process in the falling liquid film, using the measurement of the falling film interface temperature. The simulation results agreed well with the experimental results for various liquids at low flow rates. The discrepancy increases as the flow rate increases due to the limitations of the laminar smooth film model in the numerical simulation. With a volumetric concentration of ϕ=4% nanoparticle Al2O3, the heat transfer coefficient of nanofluid water, EG, LiBr-H2O increased around 15%–20% compared with the baseline fluid. A new correlation was obtained to predict the average heat transfer coefficient based on experimental data. The experimental results have an uncertainty at 9.3% for the surface temperature and 5.5% for the falling film bulk temperature. The maximum deviation between the calculated heat transfer coefficient by the model and experimental data is around 25%, at Re<1000. The average error has decreased by 23% when comparing the new correlation considering Ka and the conventional form.

Influence of Buoyancy and Inter-Surface Radiation on Confined Jet Impingement Cooling of a Semi-Cylindrical Concave Plate

Abstract In this article, the confined jet impingement cooling of a semicylindrical concave plate is analyzed numerically. The finite volume approach is applied to two-dimensional numerical simulations in the transient regime. Air is employed as the working fluid and is regarded as nonparticipant for radiation. The investigation is done for different jet Reynolds numbers (Rej) ranging from 100 to 1000, as the Richardson number (Ri) corresponding to this interval ranges between 0.1 and 10. For any Richardson number, the modified Grashof number (Gr*) is fixed at 105. When analyzing the impact of intersurface radiation between the target plate and confined surfaces on the overall cooling performance, three emissivity values (ε= 0.05, 0.5, and 0.95) are taken into consideration. Additionally, simulations are done for the pure convective heat transfer, ignoring intersurface radiation (ε= 0.0). The influence of surface emissivity and the Richardson number on velocity, temperature, and pressure distribution in the flow domain, local dimensionless temperature (θ) alterations on the target plate and confined walls, alterations in convective (Nuc), radiative (Nur), overall Nusselt numbers (Nuovr), pressure coefficient (Cp), and ratio of radiative Nusselt number to overall Nusselt number (Nur/Nuovr) on the target plate are highlighted. The findings demonstrate that surface emissivity has a significant influence on thermal and hydrodynamic boundary layer formation, buoyancy induced flow and heat transfer, and the proportion of intersurface radiation in overall heat transfer rises as the Richardson number and surface emissivity increase. At low Richardson numbers, the pressure in the stagnation region is greater than the atmospheric pressure. However, as the buoyancy effect increases, the pressure in the stagnation region falls below the atmospheric pressure and rises toward the exit.

Three-dimensional computational analysis of performance improvement in a novel designed solar photovoltaic/thermal system by using hybrid nanofluids

In the present study, the effect of using hybrid nanofluids on the performance of the novel solar photovoltaic/thermal (PV/T) system is analyzed for possible maximization of the overall efficiency that includes both electrical and thermal energy yields. A novel design has been performed to transfer heat from the backside of a solar collector to water. Three-dimensional (3D) Computational fluid dynamics (CFD) method was applied by using finite volume method (FVM). The governing parameters are Reynolds number (100 ≤ Re ≤ 1500), heat flux (400 ≤ q” ≤ 800) and different types of nanofluids such as single or hybrid nanofluids at different combinations. A considerable improvement is observed in the performance of the PV/T system when using hybrid nanofluids instead of pure base heat transfer fluids or single type nanofluids.

Performance comparison and suitable climatic zones of three water-mediated evaporative cooling technologies

Evaporative cooling is an alternative cooling technology to conventional vapor-compression systems with lower energy use and pollution emissions. This paper presents theoretical performance comparison for three water-mediated evaporative coolers and provides a design guideline for choosing evaporative coolers under specific climates and operating boundary conditions. Theoretical solutions of counter-flow evaporative cooling are given and verified with maximum absolute and relative errors of 0.26 K and 1.23% respectively. The heat transfer coefficient of the combined heat and mass transfer is 2.72–4.95 times than the pure heat transfer process if there were no mass transfer. Climatic dividing lines of various technologies are affected by the design parameters of the system, dropping the psychrometric chart into different applicable climatic areas. The simulation results show that the climatic areas in psychometrics where indirect outperforms direct evaporative cooling will expand when increasing the air–water ratio and heat transfer coefficient, and climatic area of series indirect evaporative cooling shrinks when increasing temperature difference. The simplified selection method using such difference in merits is verified by case study, which can serve as a useful guideline for choosing the proper evaporative cooling technology for a wide range of geographical locations.

Vapor condensation on micropillar structured surface with lattice Boltzmann method

Pure vapor condensation process and heat transfer on micropillar structured surfaces with various wettabilities are simulated numerically based on an improved three-dimensional phase-change lattice Boltzmann method (LBM). Condensed droplet dynamic behaviors containing nucleation, growth and departure are mainly explored. Two preferable droplet nucleation sites are detected successfully: at the corner between micropillar sides and subcooled substrate, and at the center of substrate. The effects of contact angle, micropillar spacing and height on droplet nucleation time, departure diameter and departure frequency are investigated quantitatively. Results indicate that as the contact angle increases, nucleation of condensate is delayed, while the departure diameter is reduced and departure frequency is increased. The increase in micropillar spacing, height and contact angle changes droplet from Wenzel state to partial wetting state. Moreover, it is also shown that the highest temperature and maximum local heat flux appear at the three-phase contact line. Despite the very complex roles of micropillar structures and wettability on condensation heat transfer, an optimal heat transfer surface with configurations of contact angle of 90°, micropillar spacing of 12 and height of 18 is presented. This study provides a fundamental understanding in regards to vapor condensation on microscale structured surface from numerical view.

Multiscale modeling of coupled thermo-hydro-mechanical analysis of heterogeneous porous media

This paper presents a numerical multiscale formulation for analysis of the transient heat and fluid flow in deformable heterogeneous porous media. Due to the heterogeneity of the media, the direct numerical simulation of the micro-structures leads to high computational costs. Hence, the multi-scale method can provide an efficient computational procedure. To this end, the first-order computational homogenization is adopted for two-scale simulation of THM problems. The governing equations of the problem contain a stress equilibrium equation, a mass continuity equation and an advection–diffusion equation in a fully coupled manner. Accordingly, the proper virtual power relations are defined as a bridging stage between the scales. The boundary conditions of the microscopic domain are specified in terms of the macroscopic information to fulfill the averaging constraints. In order to consider the transient effects, the micro-inertial terms are included in the micro-scale simulation, and then the microscopic boundary conditions are enhanced to assess the efficiency and accuracy. The macroscopic properties including the homogenized tangent operators and equivalent quantities are determined from the microscopic solution through the appropriate mathematical procedures. Moreover, an upwind finite element squared method is employed for highly advective heat transfer to achieve the accurate spatial results. Finally, several comprehensive examples are simulated to study the effectiveness and validity of the proposed computational model, particularly the boundary conditions; moreover, different case studies are discussed such as the highly advective flow, pure conduction heat transfer and a combined conduction–convection heat transfer.

Global sensitivity analysis of a pure steam dropwise condensation heat transfer model

Established heat transfer models for dropwise condensation (DWC) consider wetting behavior, surface structure and nucleation dynamics to calculate the heat flux. However, model results often deviate from experiments, in part due to uncertainties of the model input parameters. In this study, we apply quantitative sensitivity analysis to a pure steam DWC heat transfer model in order to attribute the variation of the model result to its input parameters. Four scenarios with different variations of the model parameters are discussed and sensitivity coefficients for each parameter are calculated. Our results show a high sensitivity of the model result towards the coating thickness, the contact angle and the nucleation site density, underlining the need to accurately determine these parameters in DWC experiments.

Hierarchical microcavity topography for enhancement of water vapor condensation heat transfer by regulating droplet dynamics and droplet size distribution

Manipulating droplet dynamics is of great importance in condensation heat transfer enhancement. Cavity topography was believed to be able to regulate droplet merging and self-removal via Laplace force imbalance by the cavity wall and the effect of spatial confinement and isolation. However, the size effect of the microcavity on the droplet dynamics behavior, droplet size distribution and condensation heat transfer performance are not sufficiently understood. In this paper, hierarchical microcavity-arrayed superhydrophobic surfaces with a nominal cavity diameter of 20, 35, 50 and 70 μm were fabricated. Condensation experiments were first carried out in the atmosphere under an optical microscope, and then in pure steam in a self-designed environmental chamber at 4000 Pa. Local dynamic behaviors and time-lapsed evolutions of droplets during condensation were observed. Droplet size distribution and jumping rate were statistically analyzed using ImageJ software. It was proved the smaller cavity size of 20 μm is beneficial to facilitate condensate droplet jumping and increase the density of small droplets. The surface coverage ratio on the microcavity-arrayed superhydrophobic surfaces was only half of that on the hydrophobic surface at steady state. Enhancement of pure steam condensation heat transfer can be realized by effectively regulating droplet dynamics using microcavity topographies at limited subcooling degrees.

On the impact of the heat transfer modelling approach on the prediction of EU-DEMO WCLL breeding blanket thermal performances

Abstract The Water-Cooled Lithium-Lead Breeding Blanket is a key component of a fusion power plant, in charge of ensure Tritium production, shield Vacuum Vessel and magnets and remove the heat power deposited by radiation and particles arising from plasma. The last function is fulfilled by First Wall and Breeding Zone independent cooling systems. Several layouts of BZ coolant system have been investigated in the last years to identify a configuration that might guarantee EUROFER temperature below the limit (550 °C) and good thermal-hydraulic performances (i.e. water outlet temperature of 328 °C). A research activity is conducted to study and compare different modelling approaches to simulate the heat transfer within the BZ liquid metal, assessing their impact on the numerical prediction of the WCLL blanket thermal performances. An approach will rely on the simulation of convective and diffusive heat transfer processes taking place within the liquid metal by means of a CFD tool based on the Finite Volume Method. Conversely, the other approach will roughly assume a pure diffusive heat transfer mechanism within the breeder, due to the very low velocities envisaged for its flow field. In this case the heat transfer performances will be preferably assessed by means of a commercial code based on the Finite Element Method. The analyses have been carried out with reference to the so called “WCLL BB 2018 V0.6″ equatorial cell. Advantages and issues from the thermal-hydraulic point of view are identified, the impact of the imposed boundary conditions and heat transfer properties, with the implemented correlations, on the respective results is critically discussed.

Numerical simulations of the Rayleigh-Bénard-Marangoni convections in a thin metallic layer

Within the context of nuclear power safety, the In-Vessel Retention (IVR) strategy consists in sustaining the integrity of the reactor pressure vessel by External Reactor Vessel Cooling (ERVC). However, due to the ablation of the vessel wall and the thermochemical effects, a thin metallic layer may be formed on top of the corium pool and lead to the vessel failure. In this article, numerical simulations of the thermal–hydraulic behavior of the thin metallic layer are carried out to reduce the uncertainties in the heat flux evaluation. Specifically, the Rayleigh-Bénard and Bénard-Marangoni convections are considered in the heat transfer models of the metallic layer. The TrioCFD code is validated for the Bénard-Marangoni convection, then applied into the numerical simulations of the metallic layer with different top boundary conditions: pure radiative heat transfer, and radiative heat transfer plus positive or negative Marangoni effects. The temperature profile, mean temperature and focusing effect of the metallic layer are evaluated and compared for various cases. Results show that the Marangoni effects have to be considered for small metallic layer thicknesses. Furthermore, the layer can be divided into two regions, of which the heat flux correlations can be evaluated independently, giving a better estimation of the heat flux profile.

A fully coupled thermo-mechanical analysis for the minimisation of spring-in and process time in ultra-thick components for wind turbine blades

The paper tackles the multi-objective optimisation of the cure stage of the Vacuum Assisted Resin Transfer Moulding (VARTM) process to manufacture the root insert of wind turbine blades. The aim of the study is to compare the Pareto front obtained from a pure heat transfer analysis, where temperature overshoot is an objective, with the one achieved when a coupled thermo-mechanical analysis is used, in which spring-in deflection is directly addressed. The optimisation methodology links the finite element solution of a heat transfer and a coupled thermo-mechanical problem of the cure process with a genetic algorithm suitable for multi-objective problems. The paper highlights that whilst minimising overshoot temperature is effective in reducing the spring-in, optimal design points might remain hidden. Furthermore, by showing the evolution of transverse residual stresses the paper suggests that designing cure cycles that introduce compressive stresses before vitrification is beneficial to counteract the subsequent tensile stresses generation.

Buoyant flow and instability in a vertical cylindrical porous slab with permeable boundaries

The basic stationary buoyant flow in a vertical annular porous passage induced by a boundary temperature difference is investigated. The vertical cylindrical boundaries are considered both isothermal and permeable to external fluid reservoirs. There exists a stationary parallel velocity field with a zero flow rate and pure conduction heat transfer. Its linear stability is analysed with normal mode perturbations of the pressure and temperature fields. The transition to convective instability is caused by the basic horizontal temperature gradient. Hence, its nature differs from that of the usual Rayleigh–Bénard instability. The linear dynamics of the perturbed flow is formulated as an eigenvalue problem, solved numerically. Its solution provides the neutral stability curve at each fixed aspect ratio between the external radius and the internal radius. The critical Rayleigh number triggering the instability is evaluated for different aspect ratios. It is shown that the system becomes more an more unstable as the aspect ratio increases, with the critical Rayleigh number dropping to zero when the aspect ratio tends to infinity.

Resolving the effects of local convective heat transfer via adjustment of thermo-physical properties in pure heat conduction simulation of Laser Powder Bed Fusion (L-PBF)

Numerical models, especially when validated against experimental findings, are valuable tools that can be used as a cheap and reliable alternative to the expensive and time-consuming experimental investigations. In the current work, a multiphysics model of laser powder bed fusion (L-PBF) has been developed for stainless steel SS 316-L and validated against experimental findings. By configuring the model, the effects of the inverse Marangoni flow on the melt pool morphology and the heat and fluid flow conditions were analysed. It was shown that for high surface tension sensitivities, the total heat flux through the melt pool boundaries increases. The enhanced cooling will lead to a smaller melt pool size with larger depth to width ratio and causes a relatively uniform temperature field, which reduces the role of the conduction heat transfer. Furthermore, a Nusselt number was derived to quantify the role of the Marangoni effect on the convection heat transfer from the melt pool. This expression was used to calculate an effective thermal conductivity required for a modified pure conduction heat transfer calculation. It was shown that, via using a combination of the derived Nusselt number and the concept of the effective thermal conductivity, the average melt pool temperature determined from the modified pure conduction model gets very close to that of the more advanced CFD model.

A novel 2-D ring-tubes model and numerical investigation of heat and moisture transfer around helix ground heat exchanger

As for the helix ground heat exchanger, a novel 2-D ring-tubes heat and moisture transfer model was built in this paper for the sake of efficiency and convenience. And a miniature model experiment was conducted to verify the correctness of the model. Four different soil initial water content conditions were simulated with the established model. In addition, the novel model was compared with the pure heat transfer model of which there is no moisture migration. The results indicate that increasing the initial soil water content can increase heat transfer capacity of the exchanger effectively. Three different heat transfer stages can be divided for the helix ground heat exchanger in the water flow direction. And during the design of exchanger, the proportion of entrance and exit stages should be increased for the sake of larger heat exchange capacity. Moreover, some effective measures can be conducted to increase soil water content near the entrance stage in order to avoid excessive reduction of soil water content in this area. There is a critical initial water content θcr for heat release condition to determine the applicability of the pure heat transfer model, and it’s 0.111 m3/m3 for the typical mudstone soil in Chongqing.

Investigation of Radiative Heat Transfer in Two-Dimensional Enclosures Using Wavelet Collocation Method

This paper uses a new quasi-Shannon interval wavelet collocation method (QSIWCM) to deal with pure radiative heat transfer in partitioned rectangular domains. The angular domain is discretized by the discrete-ordinate method, and the spatial domain is discretized by the QSIWCM. For some partitionable irregular enclosures, the whole domain is first divided into several rectangular subdomains and then discretized in each subdomain. Three geometries are considered to test the accuracy of the QSIWCM as applied to radiation heat transfer, including the square enclosure, the square enclosure with one upper baffle, and the square enclosure with one centered obstruction. The numerical results have a satisfactory agreement with data from the literature.

Transient heat fluxes measurement analysis from platinum based thin film gauges in open and closed cavities

Platinum thin film gauges (PTFGs) measure heat fluxes in the applications involving very short duration of the heating environment. Heat transfer measurement is the frequently used technique for measuring the surface heat flux using thin film gauges. The present investigation has been focused on the design and manufacturing methods for heat transfer gauge, their stability, and dynamic calibrations in certain situations where the heat load suddenly build up. PTFGs measure heat fluxes in heating environments applications during the very duration. The measurement for heat transfer is a technique used often with thin film gauges to measure the surface heat flux. The convection devices are regarded as the best measuring units in short-term transient temperature measurement applications and are usually used when the heat transfer mode is dominant means gas turbine engines, high speed aircraft, etc. In addition to that, there are many difficulties incurred for convection based measurement practically and few interdisciplinary research fields. A convective heat load is provided with a hot air gun to get the temperature signal. By using thin film gauge through present investigations, it is very ambitious to explore the possibility of short term conduction based transient measurements with pure conduction heat transfer mode. A simple experimental set up is used to supply the thin film gauges with heat flux, which is manually manufactured with platinum as a sensing material and quartz as a substrate material. The body's nose tip to high speed flow is expected to be the maximum heat transfer at the stagnation point. The stagnation point probes are fabricated for PTFGs, and baking material is quartz. The recorded temperature histories are compared with the experimentally recorded temperature signals from the gauges through the finite element method. The heat flux forecast was configured by using the one dimension thermal conduction equation convolution integral and by comparison with the heat input loads. This study reveals the ability of PTFGs to be used for a short period.

Thermal Performance Prediction in the Air Gap of a Rotor-Stator Configuration: Effects of Numerical Models

This study is dedicated to a numerical investigation of convective heat transfer on the rotor surfaces of a rotor-stator configuration that is typically found in large hydro-generators. The computational fluid dynamics calculations with two turbulence modelling approaches are used to predict the flow structure and heat transfer in the air gap of the rotor-stator configuration. The steady state mixing plane approach is employed at the interface to couple the rotor and stator components. Results show that the location of mixing plane interface in the air gap plays an important role in the prediction of heat transfer on the pole face. Also, it is indicated that the prediction of temperature distribution on the pole face is greatly affected by the turbulence models used. Furthermore, through a comparison between the pure convective and conjugate heat transfer methodologies, it is shown that the inclusion of solid domain into the numerical model significantly improves the thermal prediction of the solid components of the machine.

Entransy analysis of reversible thermodynamic cycles based on the conservation of entransy equation and its application

Entropy analyses and entropy generation analyses have been widely used to analyze and optimize thermal processes. For instance, the entropy generation minimization principle has been used to optimize heat transfer processes, heat transfer systems and thermodynamic cycles. However, some studies have reported that the entropy generation minimization principle cannot derive the optimal results for pure heat transfer processes and heat transfer systems without heat-work conversion processes. The concept of entransy and the principle of the least entransy dissipation-based thermal resistance were proposed for these problems, and they have been widely applied to optimize pure heat transfer processes and systems. Besides, the concept of work entransy was developed for irreversible thermodynamic cycles involving heat-work conversion processes, and it is defined as the product of the work and the temperature. This concept was then used as the entransy analysis approach to optimize thermodynamic cycles. However, previous studies applying the work entransy concept mainly focused on (1) the relation between the work entransy loss and the endoreversible thermodynamic cycle efficiency, and (2) the optimization of practical thermodynamic cycles involving irreversible processes. There have been few such studies of reversible thermodynamic cycles or combined cycles. This article analyzes reversible thermodynamic cycles and combined thermodynamic cycles using the entransy analysis method with a derivation of the conservation of entransy equation. First, the entransy conservation equation for a reversible thermodynamic process is derived, which introduces the concept of heat entransy and heat-work entransy flow. They are both process dependent physical quantities. The corresponding entransy conservation equation is then derived for a reversible thermodynamic cycle. The equation shows that there are two different conservation equations for reversible thermodynamic cycles, i.e. the energy conservation equation (1st law of thermodynamics) and the entransy conservation equation. Meanwhile, according to thermomass theory, the entransy is nothing else but the relative energy. The entransy conservation equation can be used to analyze the performance of reversible thermodynamic cycles. On the other hand, since the entropy returns to the initial value in a reversible thermodynamic cycle, an entropy analysis cannot evaluate the performance of reversible thermodynamic cycles. Second, a combined heat engine-heat pump cycle is taken as an example application of entransy analysis. In the philosophy of entransy analyses, the performance indicator of the combined cycle is its net heat entransy flow. For the properly chosen parameters, the combined cycle can be regarded as an ideal voltage transformer. Therefore, this kind of combined cycle can be referred to as a temperature transformer, which is different from the absorption temperature transformer. A diagram is introduced to show the total effect of the combined cycle to help understand the combined cycle. Finally, two practical cases, i.e. combined cycles for heating and refrigeration, are chosen to show applications of this entransy analysis approach. The concept of entransy efficiency of combined cycles is introduced to evaluate the performance of the combined cycles.