Convective Heat Transfer Behavior Research Articles

Experimental study on convection heat transfer properties in rough-walled fractures of granite: The effect of fracture roughness

The fracture-dominated convection heat transfer behavior is commonly involved in the development, utilization and storage of thermal energy in fractured rock engineering. An experimental system assembled by a peristaltic pump drive, a liquid preheater and an electric blast drying oven is developed to quantify the effect of fracture roughness on the convection heat transfer characteristics. The overall heat transfer coefficient (OHTC) and the amount of heat transfer quantity from six fracture samples with different inlet temperatures and flow rates are calculated by the data acquisition at five observation points. In general, the average convective heat transfer efficiency between water and rock decreases gradually with time, and then enters a stage of thermal equilibrium while the temperatures at the five observation points become constant. The increasing flow rate can lead to the gradual increase of the OHTC and the slowdown of its growth rate. The OHTC is negatively correlated with the inlet temperature. With the increase of fracture surface roughness, the dominant flow effect is significantly enhanced, which leads to the weakening of heat transfer characteristics and the gradual reduction of OHTC. Finally, the heat transfer quantity decreases with the increase of roughness, and exists an inflection point with the flow rate.

The investigation of anisotropic kelvin cells: Forced convective heat transfer

The skeleton shape of the Kelvin cell plays a significant role in influencing forced convective heat transfer behaviors, and it can be tailored to enhance overall heat transfer performance (OHTP). To this end, this study conducted a comprehensive experimental and numerical comparison of the Kelvin cells (KC), Kelvin cells with elliptical skeletons (ES), and Kelvin cells with reversed elliptical skeletons (RES) to elucidate the hydraulic and thermal characteristics. The findings indicated that the pressure drop and the overall Nuseelt number of the ES is 53.0 % and 8.2 % lower than those of the KC. Consequently, the area goodness factor, which serves as an indicator of the OHTP, is 105.5 % higher in the ES compared to the KC. Conversely, the RES does not exhibit a significant advantage in the aforementioned parameters. The reduced energy loss attributable to the presence of elliptical skeletons enhances the hydraulic performance of the ES. Furthermore, the smaller recirculation area on the leeward side of the elliptical skeleton promotes improved heat transfer near the heat substrate. The ES has advantage in the fin efficiency compared with KC. The 1.25 cell height is a significant reference value for the development of heat transfer device with Kelvin cell.

Numerical simulation of convective heat transfer behavior of nanofluids in elliptical microchannels

This study employs a numerical simulation approach to investigate the heat transfer performance of elliptical microchannel heat sinks (MCHS) with varying aspect ratios (AR) of the elliptical cross-section. The convective heat transfer behavior of water and water-based nanofluids containing CuO nanoparticles at different volume fractions within the MCHSs is examined. The research results indicate that at Re = 600, as the AR of the elliptical cross-section increases from 1:1 to 1.75:1, the thermal performance of the MCHS improves by 5.33%. Furthermore, with an increase in the volume fraction of nanofluid from 0 to 0.75%, the convective heat transfer coefficient of the nanofluid increases by 11.87%. Compared to the circle MCHS using water as the coolant, the MCHS with an elliptical cross-section with an AR of 1.75:1 and a nanofluid volume fraction of 0.75% can reduce the maximum temperature of the chip by 9.78%.

Hydrodynamic and thermal behavior of tandem, staggered, and side-by-side dual cylinders

This study investigates the impact of arrangement of two cylinders on their flow-induced vibrations (FIV) and heat transfer behavior at a Reynolds number of 100. Both cylinders were allowed to vibrate in two degrees of freedom (2DOF), encompassing streamwise and transverse directions. The arrangement of identical circular cylinders was varied across tandem (α = 0°), staggered (α = 30°, 45°, 60°), and side-by-side (α = 90°) configurations, at a constant center-to-center distance of 6D. The cylinders were heated at a fixed temperature to observe the forced convection heat transfer behavior under the influence of 2DOF FIV. To observe the FIV, the reduced velocity was varied from Ur = 0 (stationary cylinders) to 14. Results unveiled cylinder response sensitivity, encompassing vibration and heat transfer, with respect to reduced velocities and arrangements. Tandem arrangement exhibited the greatest vibrations for both cylinders. While lower drag was experienced in tandem for cylinder 2 (C-2), it escalated in staggered positioning. Both cylinders experienced lock-in between Ur = 6 and 8 for all arrangements, involving significant transverse vibration amplitudes. Maximum streamwise vibration reached 6.07% of the maximum transverse vibration for C-2 and 2.34% for C-1. Distinct slender “figure-8” and “oval-shaped” cylinder trajectories emerged, accompanied by diverse vorticity patterns in cylinder wakes across arrangements. For α = 60°, C-2 experienced 75.3% lower transverse vibration and 9.4% higher average Nusselt number compared to tandem setup. Overall, a pronounced correlation emerged between cylinder hydrodynamic behavior and heat transfer characteristics, evident through cylinder vibration, vortex shedding, average Nusselt number, and temperature distribution results.

Numerical study of the convective heat transfer coefficient for steel column surrounded by localized fires

The convective heat transfer coefficient hc characterizes the heat transfer capacity between the fluid-solid interface in a thermal convection phenomenon. At present, values adopted from specifications are mostly used to define the hc coefficients in localized fire researches. In this paper, the specific scenario of the steel column surrounded by fire plume is selected as a typical case to explore the surface hc change rules of the steel column under the action of the fire source at the bottom. Different from the application of the concept of adiabatic surface temperature (AST) to achieve fluid-solid thermal coupling, this paper takes the scenario with the fire source heat release rate (HRR) being 81 kW as an example and conducts a two-way coupling simulation analysis of the fire and the thermal models by means of CFD codes. First, the validity of this simulation methodology is verified by comparing the results of the numerical model and the experiment. Following this, the distribution laws of the spatial velocity and temperature fields are analyzed, and the hc coefficient of the steel column surface is explored in detail using theoretical formula and traditional empirical formula respectively. It is noted that the hc results obtained based on theoretical formula can comprehensively consider the velocity boundary layer and temperature boundary layer situations above the column surface, which is a direct result of the definition of the convective heat transfer coefficient. It is found that the hc‾ is lower at the bottom of the column and increases with height, with relative maximum value in the lower and middle parts of the column, reaching around 40 W/m2 K. The hc‾ results decrease along the axial direction, with the value of upper part of the column being about 20 W/m2 K. In contrast, the hc‾ distributions along the height obtained by the empirical formulas are similar to the distribution of fire plume velocity along the height. In other words, it considers only the effect of the velocity boundary layer, while the effect of significant changes in fluid temperature on convective heat transfer behavior is less considered. Compared to the results based on the theoretical definition, the hc‾ results obtained by empirical formulas are significantly smaller in the lower and middle parts of the column, and higher at the top of the column. Finally, a new hc‾ practical formula for the column surfaces surrounded by fire plume is proposed, which agrees well with the results based on the theoretical definition in several cases.

Heat transfer and thermal efficiency enhancement using twisted tapes in sinusoidal wavy tubes with nanofluids: a numerical study

PurposeThe purpose of the study is to propose a novel implementation of twisted tape in sinusoidal wavy-walled tubes to enhance the rate of heat transfer without compromising thermal efficiency. The study numerically investigates the fluid flow characteristics and analyzes the effect of different geometrical configurations, including wall wave amplitude, tape twist angles and nanoparticle volume fractions, on heat transfer improvement and performance factor.Design/methodology/approachThis problem is numerically investigated using computational fluid dynamics, and the method is the finite volume method. A two-phase mixture model is used for nanofluid modeling.FindingsThe study investigated the effect of wall waviness, twisted tape, and nanoparticles on forced convective heat transfer and friction factor behavior in laminar pipe flow in three different Reynolds number regimes. The results showed that implementing twisted tape in wavy tubes significantly increased the rate of heat transfer and the performance factor, with the best twist ratio between 90 and 180°. Adding nanoparticles also enhanced heat transfer and performance factor, but to a lesser extent than wavy wall-twisted tape combinations. The study suggests selecting a proper combination of wavy wall and twisted tape at each Reynolds number to achieve an optimum solution.Originality/valueTo the best of the authors’ knowledge, the implementation of the selected passive methods in sinusoidal wavy tubes has not been studied before, and no previous studies have taken into account such a mix of heat transfer improvement techniques.

Numerical validation and assessment of vertical and horizontal latent heat energy storage system during a melting process

Passive enhancement techniques have been widely adopted in thermal energy storage applications such as the phase change material (PCM) based latent heat energy storage systems to improve the thermal performance of these systems with minimal complexities. Recent studies have shown high interest in investigating the effect of different system orientations as a passive technique to improve the performance of thermal energy storage systems. This paper, thus, seeks to fill the research void in developing a numerical model that can be applied to assess the thermal effectiveness of the vertical and horizontal LHESS as a heat exchanger in addition to the melting dynamics of the PCM. This paper discusses a detailed numerical investigation using Computational Fluid Dynamics (CFD) simulation that was developed based on an experimental study. This paper displayed the numerical validation of the temperature profiles and heat exchanger effectiveness of both vertical and horizontal LHESS with insight into behaviour of the phase change material within the systems. The simulation results displayed the trends of the temperature profile and effectiveness were well captured against the experimental study. The numerical model displayed that the horizontal LHESS was more thermally effective than the vertical LHESS which was attributed to its superior free convective heat transfer behaviour and transient supply temperature of the heat transfer fluid. The horizontal LHESS reduced the PCM melting duration by 23 %, 13.3 %, and 33.3 % in comparison to the vertical LHESS for the HTF flow rates of 1.4, 0.7, and 0.35 l/min, respectively. It was observed that at all HTF flow rates, the rate of melting at the horizontal LHESS slowed down after approximately 85 % of the PCM have been melted which was caused by the absence of natural convection, resulting in the remaining PCM to melt mainly by conduction.

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.

Convective heat transfer behavior and AC dielectric breakdown voltage of electric power transformer oil with magnetic colloidal nano-fluid: An experimental study

This research reports on an experimental investigation into the alternating insulation breakdown voltage (AC) and convective heat transfer behavior of nano-oil for application in electrical transformers. The base fluid of the examined nano-oil is nitro libra type transformer oil, one of the common mineral oils used in transformers. It contains iron oxide magnetic colloidal nanoparticles. Upon fabrication of the experimental apparatus in the lab, an experimental study on convection heat transfer is performed under laminar flow conditions and with a continuous heat flux applied to the wall. BA100 breakdown voltage measurement instrument based on the IEC 60156 standard is also used for alternating breakdown voltage testing. The current findings from the experimental investigation of convective heat transfer are compared with and verified by the experimental results available in the literature. The experimental results demonstrate that the convective heat transfer coefficient of the iron oxide magnetic nanoparticle-prepared nano-oil is on average 4.51% higher than that of base oil. These particles have an average size of 23 nm and a volume concentration of 0.1%. Additionally, compared to base oil, nano-oil with a volume concentration of 0.1% has a 23.8% higher dielectric breakdown voltage.

Numerical modeling for thermal behavior of nanomaterial laminar flow and convective heat transfer in appearance of magnetic field

The new computational approach has been incorporated for modeling the nanofluid flow through a cavity involving a wavy inner wall which experiences uniform flux. The fluid was created by loading alumina powders with various shapes within the water. The stream function formulation was applied after employing Darcy law and final equations have been generated. The container is under the impact of Lorentz force in addition to buoyancy forces. With loading additives, Nu enhances about 17.07% and 41.57% depending on the amount of Ha. The greater value of Ha makes the effect of adding nanoparticles become more sensible. With increasing the value of shape factor, Nu increases about 11.76%. As Ha grows, the rate of cooling decreases about 41.31%. With augment of Ra, Nu enhances about 50.41% in absence of Ha.

Comparative studies on convective heat transfer behaviour of nanofluids under turbulent flow with inserts

ABSTRACT In this paper, the thermal performance of novel spiraled rod inserts in the turbulent flow of Al2O3/water, CuO/water, and TiO2/water nanofluids through a circular tube with constant heat flux boundary conditions is compared. The nanofluids used in this study contain particle volume concentrations of 0.25 and 0.5%. According to the study's findings, nanofluids have a greater Nusselt number than deionised water. Among the nanofluids used, CuO nanofluid with a spiraled rod insert of the lesser pitch gives the maximum enhancement in Nusselt number, and the enhancement value is 49.6% for 0.5% volune concentration compared to deionised water. The increase in friction factor for the above condItion is 17.59%. The maximum thermal performance factor value with a pitch of 30 mm spiraled rod insert and 0.5% volume concentration of CuO nanofluids is about 1.41.The novelty of this present study is the simple and easily manufactured inserts used to enhance heat transfer.

Numerical investigation on turbulent mixed convective heat transfer of CO2 in a horizontal miniature tube at supercritical pressure

Mixed convective heat transfer behaviors of turbulent supercritical CO2 (SCO2) flowing in a horizontal miniature tube (di = 2 mm) have been investigated using an in-house code. The computational model adopted was verified against experimentally determined data with a number of operating conditions. The numerical simulations were carried out for pressure of 8 MPa, inlet Reynolds number of 42,521, and wall heat fluxes ranging from 50 to 320 kW m−2. A significant influence of natural convection in horizontal forced turbulent SCO2 flow was discerned, despite the small diameter and high turbulence intensity. The mechanism of mixed convection heat transfer was studied by analyzing the cross-sectional distributions of thermal properties, eddy diffusivity of heat, and secondary flow vectors. The effect of wall heat flux on the non-uniformity in heat transfer was further investigated, covering q/G ratios ranging from 41.67 to 266.67 J kg−1. The regime of heat transfer was identified by comparing the numerical results with the reference Nusselt number calculated using the Dittus-Boelter correlation. A transition from enhanced to deteriorated heat transfer regime, as well as subsequently heat transfer recovery was found with increasing bulk fluid temperature. In addition, the applicability of three existing buoyancy parameters (Buc, BuJ, and BuP) was evaluated. The Petukhov buoyancy criteria agreed best with the simulation results. A new threshold of 6.0 for BuP was determined by performing a comparative study with cases without gravity, above which natural convection would have a considerable influence on forced turbulent heat transfer.

Analysis of Convection Phenomenon in Enclosure Utilizing Nanofluids with Baffle Effects

The behavior of convective heat transfer in an enclosure filled with Cu–water nanofluid with a baffle has been numerically studied using the finite element method. The enclosure’s top and bottom walls were adiabatic, while the other two were maintained at various temperatures. The left hot wall had an effective thickness and a baffle was added to the bottom wall. The influence of different parameters like the nanoparticle’s concentration (ϕ), Rayleigh number (Ra), the thermal conductivity ratio of the thick wall (Kr), baffle angle (Ø), and the hot wall thickness (D) on the isotherm and fluid flow patterns were examined. The result showed that the average Nusselt number was enhanced, owing to the strength of the buoyancy force becoming more effective. Furthermore, as the baffle inclination angle increased, the maximum stream function at the core corresponded to the angle when it reached Ø=60°, then it gradually decreased to the minimum value as the baffle angle reached close to Ø=120°.

Numerical Investigation of Seepage and Heat Transfer in Rocks with Various Fracture Patterns for Geothermal Energy Extraction

Unraveling the seepage and heat transfer coupled process between the working fluid and the fractured rocks in geothermal reservoir is of great significance to the exploitation and utilization of geothermal resources. In this study, based on the numerical modeling of fracture flow in geothermal reservoirs, the seepage and convective heat transfer behavior of fractured granite during geothermal extraction were investigated, and the effects of different fracture patterns, fluid injection temperature, and injection velocity on the temperature evolution of rock mass were comparatively analyzed. The results clearly revealed the influence of five different fracture patterns, i.e., single fracture, echelon fracture, parallel fracture, Y-shaped fracture, and crossfracture, on heat transfer capacity. Generally, the echelon fracture has the highest fluid outlet temperature, while the crossfracture has the largest total heat and shows the strongest heat transfer capacity. Besides the fracture shape, the fluid injection temperature and injection velocity also play significant roles in the heat transfer performance. The increase of fluid injection temperature would improve the total outlet heat of the crossfracture system and also benefit the system life. When the fluid injection temperature raises from 20°C to 35°C, the system life and the total outlet heat would be increased by 58.76% and 22.42%, respectively. However, higher fluid injection velocity would damage the system life of the geothermal reservoir, which obviously results in a decrease in total outlet heat. With the fluid injection velocity increasing from 0.004 m/s to 0.006 m/s, the system life could drop by 72.47%, and the total outlet heat was reduced by 55.72%. This work contributes to the preliminary understanding of the coupled seepage and heat transfer behavior in rock mass with various fracture patterns, and it could provide some practical implications for the rational exploitation of geothermal resources.

Turbulent convective heat transfer behavior of supercritical water flowing upward in 2 × 2 rod bundle channels with various spacers

The turbulent flow and heat transfer characteristics of supercritical water, flowing in vertical 2 × 2 rod bundle channel, are numerically investigated to find useful methods to improve the cooling performance in the core of a supercritical water-cooled reactor. The heat transfer behavior as a function of mass flux and inlet temperature is analyzed and discussed. Standard grid and helical wire spacers are added to the channel, and overall heat transfer performance is evaluated and compared. The results indicate that the fuel rods can be cooled more effectively when the fluid temperature is near the pseudocritical point due to the high specific heat capacity at this temperature and pressure, and the local heat transfer coefficient can be improved greatly from the application of grid spacers; however, heat transfer enhancement only happens in limited regions downstream of the grid spacers. By contrast, strong swirl flow can be induced by a helical wire, thus the mixing of coolant is strengthened, and the convective heat transfer is enhanced. However, local hot spots also occur in the region near the wires. Both grid and helical wire spacers will lead to an increase in pressure loss, but the additional pressure drop induced by the helical wire is smaller, hence better comprehensive heat transfer performance is achieved for a wire-wrapped channel. Moreover, heat transfer performance can also be enhanced by reducing the pitch of the spacers. This suggests that applying spacers and optimizing their configurations and structures is helpful to achieving better cooling performance in rod bundle channels.

Effect of metal oxide particles on the flow and forced convective heat transfer behaviour of microencapsulated PCM slurry

ZnO, nano ZnO and nano Al2O3 were mixed with microencapsulated phase change material slurry (MPCS) for improving the heat transfer performance of slurries in this paper. The thermal and rheological properties of MPCS were measured using DSC, thermal conductivity meter and rheometer. The results show that the thermal conductivity of 5 wt% MPCS with 1 wt% ZnO, nano ZnO and nano Al2O3 was 17.9 %, 19.4 % and 23.5 % higher than that of 5 wt% MPCS, respectively. The forced convection heat transfer experiment of slurries was carried out in a loop system with various heat flux and flow conditions. The influences of heat flux, flow rate and metal oxide particles on the flow and heat transfer behaviour of slurries were investigated. The results show that the heat transfer was significantly enhanced for all slurries with metal oxide particles under three flow conditions. Compared with water, the local heat transfer coefficient (hx) of MPCSs with 1 wt% ZnO, nano ZnO and nano Al2O3 increased by 6.5 %, 9.1 % and 12.4 % under laminar flow, 6.6 %, 15.5 % and 14.9 % under transition flow, and 15.7 %, 19.0 % and 21.6 % in turbulent condition, respectively.

Study on the fluid flow and heat transfer characteristics of a horizontal elliptical cylinder under thermal buoyancy effect

Thermal buoyancy can be used as a tool for flow control and vortex shedding elimination. In the present study, a numerical investigation of the thermal buoyancy effect on the laminar flow and heat transfer characteristics of an elliptical cylinder with an isothermal surface was performed. The Richardson number varied from 0.0 to 1.5 for each angle of attack, while the Prandtl number is constant at 0.71. The Reynolds number was 1000 based on the major diameter of the cylinder, with an axis ratio (minor to major diameter ratio) of 0.25. The momentum and temperature equations were solved using a pressure-based in-house code with a second-order temporal and spatial accuracy. An initial examination showed that the thermal buoyancy led to the elimination of the vortex shedding or decrease in the flow separation area and also improved the aerodynamic performance of the cylinder. Additionally, it was discovered that insulation of the underside of the cylinder improved aerodynamic performance while reducing thermal energy consumption when compared to a fully isothermal cylinder. Other studies' findings indicated that the drag coefficient increased as the angle of attack increased; however, the Richardson number had little effect on the drag coefficient. The considered problem exhibited three distinct flow patterns: a vortex shedding region, a steady separated flow, and attached flow. Furthermore, a study on the heat transfer characteristics revealed that increasing the Richardson number increased the mean Nusselt number; however, for α > 15o, the convection heat transfer behavior was altered.

Thermal analysis of simultaneously evolving laminar pulsatile flow through two large parallel plates with time-dependent heat flux boundary conditions

Understanding unsteady convective heat transfer (UCHT) behavior of simultaneously evolving laminar pulsatile flow between two large parallel plates exposed to time-dependent thermal boundary conditions is the goal of this research. The time-dependent sinusoidal velocity waveform is applied at the entrance of the two large parallel plates exposed to time-dependent sinusoidal heat flux boundary conditions (TSHBC). The finite volume technique is used in the present work. The current results have been compared to the literature data under the same operating conditions and found to be in good concurrence. The influence of various parameters, for instance, amplitude, frequency, phase difference, heat flux ratio, and the Reynolds number on the transient Nusselt number, is investigated. An improvement in energy transfer is observed when the pulsatile flow between two large parallel plates with TSHBC than the non-pulsatile flow with a fixed heat flux type boundary. Furthermore, the energy transfer improvement is observed once the velocity and heat flux waveform frequency is equal. The applicability of the present work could be found in the solar heating system, cooling of electronic devices, etc., whereby a more robust understanding of the unsteady convective heat transfer could facilitate the design and develop an efficient system.

Convective heat transfer of water flow in intersected rock fractures for enhanced geothermal extraction

Numerous intersected rock fractures constitute the fracture network in enhanced geothermal systems. The complicated convective heat transfer behavior in intersected fractures is critical to the heat recovery in fractured geothermal reservoirs. A series of three-dimensional intersected fracture models is constructed to perform the flow-through heat transfer simulations. The geometry effects of dead-end fractures (DEFs) on the heat transfer are evaluated in terms of intersected angles, apertures, lengths, and the connectivity. The results indicate that annular streamlines appear in the rough DEF and cause an ellipse distribution of the cold front. Compared to plate DEFs, the fluid flow in the rough DEF enhances the heat transfer. Both the increment of outlet water temperature Δ T out and the ratio of heat production Q r present the largest at the intersected angle of 90° while decline with the decrease of the intersected angle between the main flow fracture (MFF) and the DEFs. The extension of the length of intersected DEFs is beneficial to heat production while enhancing its aperture is not needed. Solely increasing the number of intersected DEFs induces a little increase of heat extraction, and more significant heat production can be obtained through connecting these DEFs with the MFF forming the flow network. • Coupling hydrothermal behaviors in three-dimensional intersected plate and rough fractures are compared. • Fluid flow in intersected rough dead-end fractures can enhance the heat recovery in the main flow fracture. • Influences of several geometries parameters of intersected dead-end fractures on the heat transfer are evaluated.

Study on Convective Heat Transfer of Supercritical Nitrogen in a Vertical Tube for Liquid Air Energy Storage

The convective heat transfer behavior of supercritical nitrogen (S-N2) has played a significant role in optimizing the design of recently emerging cryogenic cold storage and recovery systems. However, studies on S-N2 heat transfer have been relatively scarce, not to mention that there is a legitimate urge for a robust numerical model to accurately predict and explain S-N2 heat transfer under various working conditions. In this paper, both experimental and numerical studies were conducted for convective heat transfer of S-N2 in a small vertical tube. The results demonstrated that the standard k-ε model performed better for predicting the key heat transfer characteristics of S-N2 than the SST k-ω model. The effects of heat flux and inlet pressure on the heat transfer characteristics under a large mass flux were evaluated. The variation mechanisms of local heat transfer performance were revealed by illustrating radial profiles of thermophysical properties and turbulent parameters of N2. It was found that the local performance variation along the flow direction was mainly determined by the radial profile of specific heat while the variation of the best local performance with the ratio of heat flux to mass flux was mainly determined by the radial profile of turbulent viscosity.