Local Heat Transfer Performance Research Articles

CFD simulation of novel adaptive pin-fins microchannel heat sink to improve thermal management of electronic chips

Frequent hotspots pose a significant challenge for chip thermal management. Conventional microchannel with fixed structure cannot deal with random hotspots, which appear in irregular time and space. To address random hotspots and reduce microchannel energy consumption, a novel adaptive pin-fins microchannel (A-MCHS) is proposed in this paper to intelligently tackle with the hotspot region, which consists of a number of pin-fins with property of phase transition and volume shrinkage when the temperature is raised to 313.15 K. A CFD method is applied to study its thermal performance and user defined function (UDF) is used to realize the adaptive thermal response of pin-fins. It’s found that the adaptive pin-fins of the hotspot undergo volume reduction and enlarged local coolant flow under higher hotspot heat flux, illustrating the automatic and adaptive flow regulation effect. Compared with the fixed pin–fin microchannel, the local heat transfer performance of A-MCHS in the hotspot region is enhanced by 37.9 %, average Nu of the microchannel is advanced by 7.4 %, and the maximum chip surface temperature is reduced by up to 3.12 K. Moreover, it also shows a significant reduction of 10.7 % in pressure drop, suggesting a great energy saving in pumping cooling fluid. It’s indicated that the adaptive pin-fins microchannel provides efficient cooling and intelligently enhanced thermal management and energy saving for high heat flux electronic chips.

Simulation on the performance and cooling capacity compensation solution of a loop thermosyphon system under fan failure conditions

Fan failure of loop thermosyphon systems (LTS) is a major factor affecting thermal safety of data centers. This paper develops a distributed-parameter model to accurately predict the overall and local heat transfer performance of a loop thermosyphon (the terminal equipment of LTS) under fan failure conditions. The simulation results show that the positions of faulty fans have a significant influence on the evaporator's local airside heat transfer coefficient, but have slight influences on the evaporator's local refrigerant heat transfer coefficient and the condenser's overall heat transfer coefficient, which would not increase the risk of heat transfer performance. Thus, fan airflow rate backup can be used to handle fan failure. Then, the effect of operating parameters including backup airflow rate (Bfan), cold water temperature (Tcw) and increment of cold water flow rate (Im,cw) on the effectiveness and efficiency of the cooling capacity compensation under fan failure conditions is analyzed by using the Taguchi method with an L16 orthogonal array. The Bfan is the primary factor contributing 81.51% on cooling capacity compensation, with Tcw and Im,cw contributing 15.25% and 2.38%, respectively. Considering the energy efficiency, the order of adjustment should be Bfan first, Im,cw second, and Tcw last when fan failure occurs.

Numerical investigation on flow characteristics and thermal performance of phase change material in spherical capsules with smooth or dimpled surfaces

The utilization of phase change spherical capsules with dimpled surfaces in a packed-bed latent thermal energy storage (PLTES) system provides a novel approach to addressing the intermittency and fluctuation in solar thermal utilization processes. In the context of accounting for wall effects, a comparative analysis of the flow characteristics and thermal performance of smooth and dimpled spherical phase-change capsules is conducted using the Large Eddy Simulation (LES) and an enthalpy-porosity method. Results show that the dimpled spherical capsule exhibits a substantial improvement in thermal performance compared to the smooth spherical capsule, with a maximum increase of 28.3 % in time-averaged Nusselt number when the difference in time-averaged drag coefficient is within 2 %. Compared to the smooth spherical capsules, the transient characteristics of dimpled spherical capsules exhibit significant changes, with the fluctuation amplitude of the drag coefficient and the fluctuation frequency of the lift coefficient both increasing by approximately twice. The local heat transfer performance also exhibits significant differences. The increase in surface temperature enhances the melting rate of PCM inside spherical capsules, but with the temperature rise, the enhancement efficiency decreases. The dimpled structure has a promoting effect on the melting process of PCM inside spherical capsules, and with the increase in surface temperature, this promoting effect becomes more pronounced. The time for PCM to achieve complete melting can be reduced by up to 19.07 %. The work contributes a novel idea for the application of non-smooth structures in PLTES systems.

Effect of truncated pin-fins on flow and heat transfer characteristics with impinging jets on a concave surface

The double-wall cooling system containing jet impingement and full-length pin-in cooling structures shows good performance in heat transfer enhancement and uniformity, which has-been used in the blade cooling design. However, the pressure loss induced by the full-length pin-fin is relatively high. It is very meaningful to get a cooling type which can balance the heat transfer performance of double-wall cooling and flow resistance, especially for the blade leading edge. In this study, numerical simulations have been carried out to study the influences of truncated pin-fins height and arrangement on heat transfer increment and flow behavior of jets impinging onto a concave double wall channel with varying truncated pin-fins arranged, considering the heat transfer enhancement potential and relatively weaken pressure loss penalty of truncated/broken pin-fin. Jets impingement on a smooth-concave surface is regarded as Baseline. Streamlines in two-dimensional sections in the channel and limiting streamlines near the target wall conjugated with flow characteristics figures are applied to better understand the flow structures in the channel. The local and averaged heat transfer performance is analyzed based on the flow discussions. The overall parameters, i.e., thermal performance, friction factor and pumping power, are also obtained. The results show that the introduction of relatively high truncated pin-fins located on the upper wall presents higher heat transfer enhancement and lower pressure loss comparing with the full-length case and truncated pin-fins located on pressure loss comparing with the full-length case and truncated pin-fins located on target wall cases. The truncated pin-fins placed on the upper surface with relative height of 0.75 reaches the highest overall thermal performance and uniformity in this work.

Thermohydraulic and Irreversibility Analyses of Swirl Flows Utilizing Distorted Radial Fins: Entropy Generation and Entransy Dissipation Evaluation

The current investigation analyses the convective heat transfer performance, entropy generation, and entransy evaluation of swirl flows generated by distorted radial fins (DRF). As swirl flows and various vortical structures induce large temperature gradients, second law and entransy analyses are necessary to thoroughly evaluate their true thermodynamic influence on heat transfer enhancement. The results indicated that due to the influence of swirl flows and vortices, all angles of the DRF were capable of inducing intense fluid mixing, thinner thermal boundary layers, and turbulent eddies. It was found that overaggressive swirl flows may hinder local heat transfer performances, by enclosing low-velocity heated fluids within the thermal boundary layers. However, as these overaggressive swirl flows and strong vortices propagate downstream, beneficial fluid mixing was eventuated, favouring heat transport over large regions. In terms of thermal performances, the maximum heat transfer enhancement was exhibited by the α=45° DRF, improving the Nusselt numbers up to 59.3%. Accordingly, the highest performance evaluation criterion (PEC) of 1.269 was obtained by the α=45° DRF at the Reynolds number 2389, attributed to the centrifugal effects of the swirl flows. Optimal entropy generation numbers were also exhibited by the α=45° DRF at the highest studied Reynolds number, reducing total entropy generation by 36.81%. Lower entransy thermal resistances were also accredited to greater DRF angles due to the intense swirl effects. In essence, the study concludes that the effects of swirl flows and vortices significantly enhance heat transfer, whilst reducing both entropy generation and entransy dissipation rates, leading to optimal thermal performances.

Theoretical and numerical study on new evaluation criteria for longitudinal vortex enhanced heat transfer

The enhancement of heat transfer caused by longitudinal vortex phenomena has been a widely studied topic for a significant period in fluid mechanics and heat transfer. However, despite extensive efforts, the current understanding of longitudinal vortex flow and the involved physical mechanisms leading to enhanced heat transfer remains qualitative in nature. To unveil the physical mechanisms underlying the enhancement of convective heat transfer by longitudinal vortices, further quantitative research and in-depth discussions are necessary. By performing detailed derivations and proofs of the equations, this study aims to further understand the relationship between local vortex variations and heat transfer performance. The results indicate that the presence of longitudinal vortices alters the gradient of the convective term and ultimately leads to enhanced heat transfer performance. Based on the derived results, corresponding metrics have been proposed to quantitatively analyze the degree of heat transfer enhancement due to longitudinal vortices. This proposed quantitative analysis metric has been applied to evaluate the heat transfer performance of a cross-wavy primary surface heat exchanger, validating the reliability of the metrics. The research content and conclusions provide valuable insights for both theoretical investigations and practical engineering applications in the field of enhanced heat transfer.

Influence of cellulose nanofiber fluid on flow instability and heat transfer of two-phase closed thermosyphon

In this research, the geyser phenomenon occurring in a small-diameter two-phase closed thermosyphon (TPCT) was observed and the instability of the device was discussed. Geyser phenomena interfere with the natural circulation of internal working fluids, increasing the thermal resistance of the system and contributing to the instability of the device. This study attempts to improve the thermal performance and stability of the system using cellulose nanofiber (CNF) fluid as the working fluid. The use of CNF fluid was observed to reduce the magnitude of temperature change inside the evaporator of the TPCT significantly. Moreover, it improved the local boiling heat transfer coefficient by 3.1 %, 87.3 %, and 181.2 % on average when the filling ratios are 0.25, 0.5, and 0.75, respectively. Studying the local heat transfer performance and instability will be helpful in designing a more stable TPCT efficiently. Additionally, the findings of this study can be applied to solar thermal power generation or heat pipe research for cooling strategies in computing servers, depending on the input heat load.

Experimental and numerical investigations on thermal-hydraulic characteristics of supercritical CO2 flows in printed circuit heat exchangers

In the supercritical CO2 (SCO2) Brayton cycle, the pre-cooler is employed to cool SCO2 to be near the pseudo-critical point state to minimise compression work and achieve high cycle efficiency. The dramatic variation in thermophysical properties of SCO2 makes heat transfer and flow very complex in the pre-cooler, bringing a lot of difficulty to the design of the related components and system. This study attempts to address this issue. The experiment and simulation are carried out to analyse the influences of mass flux, inlet temperature, and pressure on the thermal-hydraulic characteristics of SCO2 flows in a straight-channel printed circuit heat exchanger (PCHE) employed as a pre-cooler, and the underlying heat transfer mechanism is analysed as well. The local heat transfer coefficient (h) is higher when the phenomenon that the local temperature of SCO2 (Tf) is equal to the pseudo-critical temperature (Tpc) appears in the laminar sublayer than when this phenomenon appears in the buffer layer. The new heat transfer and flow correlations are presented, and the genetic algorithm and back-prediction (GA-BP) neural network is employed to predict the local heat transfer and flow performance. The maximum relative error between the Nusselt number (Nu) predicted by the new heat transfer correlation and Nu obtained via experiment and simulation is within ±20%. The maximum relative error between the Nu predicted by the GA-BP neural network and the Nu obtained through experiment and simulation is within ±15%. The maximum relative error between the Fanning friction factor (f) predicted by the new friction factor correlation and f obtained via experiment and simulation is within ±5%. The maximum relative error between f predicted by the GA-BP neural network and f obtained through experiment and simulation is within ±4%. The present investigations analyse the thermal-hydraulic characteristics of SCO2 in the pre-cooler prototype and could provide useful guidance for the design of pre-coolers for the SCO2 Brayton cycle.

Numerical analysis of the flow and heat transfer characteristics of oil-gas-water three-phase fluid in corrugated plate heat exchanger

The heat exchanger is the key device for waste heat recovery from oilfield wastewater, which is an oil, water, gas mixture. Therefore, this paper aims to numerically investigate the flow and heat transfer performance of the corrugated plate heat exchanger by using the oil-gas-water three-phase mixture as the working flowing. The multi-fluid Eulerian framework along with the population balance model is adopted and validated first. Then the effects of different factors on the flow and heat transfer performance of a corrugated channel are studied. The results indicate that the gas accumulation on the wall will deteriorate the local heat transfer performance of the corrugated channel. Increasing the inlet gas content could enhance the gas holdup near walls and decrease the local heat transfer performance, as well as the comprehensive heat transfer performance (PEF) of the corrugated channel. The corrugation height remarkably influences the average Nu and pressure drop of the corrugated channel. When the Re is small (Re < 3000), the corrugated plate channel with a large corrugation height exhibits the best heat transfer performance; however, when Re is large, the heat transfer decreases with the rise in the corrugation height. Besides, the results demonstrate that the inlet gas bubble size has almost no influence on the heat and flow performance of the present corrugated plate sewage heat exchanger.

Heat transfer characteristic of particle flow around the out-wall of different geometries

In this study, large-scale simulations were carried out for different tube banks to explore the effect of geometrical factors. In addition, the heat transfer experiment of dense particles was carried out to validate the heat transfer model used in discrete element method. The stagnation zone and void zone with nearly triangular shape are found in staggered tubes. The stagnation zone with rectangular shape is formed between vertical adjacent tubes of the aligned tube, and the width of stagnation zone is one third of the tube diameter. A triangular particle blockade zone is formed in the rectangular stagnation zone. The heat transfer rate of staggered tubes is increased by 26% compared with that of the aligned tubes. The staggered elliptical-like combined tubes bank with chamfers significantly increase the heat transfer rate by 54.2% with the same heat transfer coefficient, compared with the staggered circular tubes bank. The local heat transfer performance can be enhanced by the addition of chamfers at the junction area of tubes. When penetration thermal resistance is dominant compared with the contact thermal resistance, the circular tube has higher heat transfer coefficient compared with elliptical-like combined tube with chamfers. The thermal resistance models for different tubes are developed.

Nano-jet impingement cooling of a sinusoidal wavy hot surface by using an oriented L-shaped object under magnetic field with two phase mixture formulation

In this study, slot nano-jet impingement cooling of a wavy sinusoidal surface is analyzed by combined utilization of an oriented L-shaped object and magnetic field. A two phase mixture model of nanofluid is adopted. It is observed that the presence of the L-shaped object and its orientation are very influential on the local and average heat transfer performance. Magnetic field suppresses the vortices near the object and on the bottom all. The average Nusselt number (Nu) rises by 17.5% and 8.6% when its strength is varied for object orientations of te=−90 and te=90. When magnetic field inclination is concerned, the cooling performance improvements are in the range of 25.6%–53.9% depending upon the orientation of the object. The orientation of te=0 and te=45 are the most favorable cases in terms of thermal performance while significant variations in the average Nu are seen in the absence of magnetic field when varying the L-shaped object orientation. When compared to reference case of slot jet impingement (without magnetic field using pure fluid and without object), using nanofluid improves the cooling performance by about 8.4%. When object is installed at Ha=0, it rises by 27% at orientation of te=45. When magnetic field is used, additional rise of thermal performance with nanofluid is obtained up to 10% when compared to case by using only pure fluid.

Experiment on local convective heat transfer for successive droplets impacting on heated cylindrical surface

An experiment was conducted to investigate the underlying heat transfer mechanism when successive droplets impacting on the heated cylindrical surface. The hydrodynamics of droplets were captured by visual method and the local convective heat transfer characteristics were obtained by combining direct measurement and numerical solution of three-dimensional differential equation of heat conduction. The influences of impact frequency, impact velocity and input heat flux were experimentally discussed. It is found that, based on the evolution of local wall temperature, there are three zones along the circumference, i.e., impingement zone, heat diffusion zone and tail detachment zone. Increasing the impact frequency and input heat flux enhances the convective heat transfer performance. However, when the impact frequency is high enough, continuously increasing the impact frequency, the degree of convective heat transfer improvement gradually decreases. The increase of impact velocity mainly enhances the convective heat transfer on the impingement zone and heat diffusion zone, but has little effect on the tail detachment zone. The continuous increase of impact velocity results in the splashing phenomenon and the decrease of convective heat transfer performance. Furthermore, some dimensionless correlations were proposed for predicting the droplet splashing threshold and the local convective heat transfer performance along circumference, respectively.

Numerical analysis of local flow heat transfer of supercritical LNG across the pseudophase transition in different airfoil channels

As the core equipment of floating storage and regasification unit (FRSU), the function of liquefied natural gas(LNG)vaporizer is to heat the LNG from -162 °C to normal temperature under supercritical pressure, which is a pseudophase transition process. This paper numerically investigates the local flow and heat transfer of supercritical LNG during the pseudophase transition process of three airfoil channels with different structures and shapes under transcritical temperature condition. The secondary flow generated by the shape and structure of the airfoil has an important influence on the local flow heat transfer performance. Especially in the liquid-like region, the three airfoil fins not only generate mixed disturbance of the velocity vector in the y-direction, but also generate secondary vortices in the z-direction, which can more effectively interfere with the boundary layer and enhance the heat transfer performance. As the temperature increases, the density and the viscosity of the supercritical LNG in the pseudo-critical and gas-like regions decrease, the vortexes formed in the three airfoil fins appear above or below the tail of the airfoil. The research shows that under the selected conditions, Fin-2 has the best hydraulic performance, especially in the gas-like region, and its f-factor can be lower than 0.02. Because Fin-2 changes the curvature of the leading edge and minimizes the impact surface, it is conducive to the rapid discharge of local fluid, thus reducing the flow resistance and generating secondary flow at the tail of the airfoil. Fin-3 has the best heat transfer performance, and its j-factor is 5–10% higher than that of other airfoil fins. This is the tail shape of the airfoil is concave, and the secondary vortices are located in the concave area, which disturbance the boundary layer and improve the heat transfer process.

Thermal Visualization and Performance Analysis in a Channel Installing Transverse Baffles with Square Wings

The experimental examination of local heat transfer, thermal intensification, friction factors, and thermal performance factors (TPF) in a rectangular channel with square-winged transverse baffles (SW-TB) are presented in this paper. The purpose of this study is to modify the typical transverse baffles (TB) into square-winged transverse baffles (SW-TB) in order to improve the thermal performance and heat transfer rate of the channel. The effects of SW-TBs with various wing attack angles and Reynolds numbers on the heat transfer performance characteristics were examined using a thermochromic liquid crystal sheet. In the experiments, the SW-TBs were attached to the bottom wall of the channel, which had an aspect ratio (W:H) of 3.75:1. The SW-TBs had a width (w) of 150 mm, a square perforated cross-sectional area (a × b) of 8 × 8 mm2, and attack angles (θ) of 0° (solid transverse-baffle), 22.5°, 45°, 67.5°, and 90°. The bottom wall of the channel was evenly heated, while the other walls were insulated. The temperature contours on the heated surface were plotted using temperatures obtained through using the thermochromic liquid crystal (TLC) image-processing method. Experimental results revealed that the SW-TBs created multiple impinging jets, apart from the recirculation. At the proper attack angles (θ = 22.5° and 45°), the SW-TBs offered greater heat transfer rates and caused lower friction losses, resulting in higher TPFs than the solid transverse baffles. In the current work, channels where the SW-TBs display a θ = 45° presented the greatest TPF, as high as 1.26. The multiple impinging jets issuing by the SW-TBs suppressed the size of the recirculation flow and allowed better contact between the fluid flow and channel wall.

Investigation of fluid flow and heat transfer in a narrow channel with micro barchan-dune-shaped humps

• A bionic heat sink with micro barchan-dune-shaped humps is proposed. • The micro dune can reinitialize the boundary layer by inducing a secondary flow. • The micro dune generally outperforms traditional fins in overall heat transfer performance. This paper proposes an innovative bionic heat sink with micro barchan-dune-shaped humps for improving the flow and heat transfer performance of the smooth narrow channel. The effects of the Reynolds number of the main flow ( Re ) and pitch distance of the dune array ( P ) on the thermal-hydraulic performance are investigated. In this work, the air is the working flow. The Reynolds number range is considered to cover the regimes of laminar, transitional, and turbulent, Re = 860–6,050, and the pitch distance ( P ) equals 8, 12, and 16 mm. The results show that the micro barchan-dune-shaped hump can induce a secondary flow downstream of the slip face and enhance the local heat transfer performance. For the multi-dune models, the case with P = 16 mm provides the highest overall thermal performance parameter ( η ). The η value decreases monotonically with decreasing P in the considered range. In addition, compared with other micro pin fin shapes (including the hexagonal cylinder, square cylinder, and triangular cylinder), the η of the barchan dune shape is almost identical to that of the hexagonal cylinder and exceeds the other two shapes by more than 20%.

Optimization of zigzag parameters in printed circuit heat exchanger for supercritical CO2 Brayton cycle based on multi-objective genetic algorithm

The printed circuit heat exchanger (PCHE) shows promise for applications in the supercritical carbon dioxide (S-CO2) Brayton cycle due to its great resistance to high temperature and pressure. The zigzag channel is the typical channel configuration of PCHEs containing structural parameters such as pitch, bend angle, and bend radius that can significantly influence flow and heat transfer performance. In this study, the pitch, bend angle, and bend radius are selected as the design variables, while the pressure and heat exchange capacity are considered as the optimization objectives. Based on the non-dominated sorting genetic algorithm-II (NSGA-II), optimization work is conducted to improve the performance and size of the zigzag channel over a wide range of structural parameters. To predict local flow and heat transfer performance, the surrogate model embedded in the one-dimensional analysis code is proposed using the artificial neural network (ANN) method and trained by 144 groups of three-dimensional simulation results. The results calculated by the one-dimensional analysis code show that both the bend angle and bend radius can significantly affect the performance of the printed circuit heat exchanger, while the bend radius plays a more important role on the pressure drop. However, the effect of the pitch on the zigzag channel is relatively small. The optimization results indicate that the appropriate core length of the zigzag channel varies with the design of the outlet temperature. For the 0.4 m core length suitable for the 306 K outlet temperature, the size is 20% less than for a 0.5 m core length with similar flow resistance, and the pressure drop is 66.3% less compared to that of the 0.3 m core length. In addition, the zigzag channel with hybrid structural parameters is proposed and compared to the conventional zigzag printed circuit heat exchanger. The optimization results indicate that the maximum difference between the two objectives is only 4%. Thus, the zigzag channel with constant structural parameters can satisfy the demand for optimization with a simpler design.

Analysis on the heat transfer performance of supercritical liquified natural gas in horizontal tubes during regasification process

The submerged combustion vaporizer (SCV) is an indispensable peak-shaving heat exchanger in the regasification process of liquified natural gas (LNG). This paper mainly studies the trans-critical heat transfer performance of LNG in the horizontal tube of SCV under the convective heating condition. To this end, the conditions of fluid temperature, mass flux, and tube diameter are comprehensively investigated through numerical simulation. Based on the simulation results, the influence of the buoyancy on total and local heat transfer performance is analyzed from the distribution of flow field and thermal-physical property in the boundary layer. The results indicate that the buoyancy leads to a significant difference in convection thermal resistance around the circumference. Tube diameter and mass flux affect the degree of heat transfer strengthening in the lower part and heat transfer deterioration in the upper part of the tube, as well as the occupied circumferential proportion of the corresponding region. Considering the effects of local heat transfer strengthening and deterioration, buoyancy shows an enhancement effect overall on the heat transfer process, especially under the conditions of low mass flux and large diameter. Based on the mechanism analysis of the convective heat transfer, two correlations are proposed to calculate the local convection thermal resistance in the buoyancy strengthening and buoyancy deterioration region, respectively.

Comparisons of Supercritical Loop Flow and Heat Transfer Behavior Under Uniform and Nonuniform High-Flux Heat Inputs

The heat transfer characteristic of supercritical water is one of the crucial issues in SuperCritical Water-Cooled Reactors (SCWRs). The efficiency and safety of the SCWR system are largely dependent on the local heat transfer performance. This paper establishes the numerical model for supercritical water in a long vertical circular loop (inside diameter = 10 mm) and analyzes the flow and heat transfer mechanism during the transition process from subcritical to supercritical states under various heat fluxes (uniform and nonuniform). The results reveal that the difference in thermophysical properties between the boundary layer and the core region is the main reason for the heat transfer behavior, especially during the transition from subcritical to supercritical and liquidlike to gaslike. The flow structure on the buffer layer is a dominating factor for heat transfer deterioration. The cases under variable nonuniform heat fluxes have a higher heat transfer coefficient compared with uniform heat fluxes. But, this will cause large changes of the parameter locally. The dominating factors of heat transfer deterioration under these conditions are also identified.

Detailed velocity and heat transfer measurements of an advanced insert for impingement cooling

This work reports the results of paired experiments for the aft section of a complex internal cooling flow within a gas turbine vane using Magnetic Resonance Velocimetry (MRV) and steady-state Infrared (IR) thermography. The aft cooling insert for the vane with two-pass impingement was designed at a scale five times larger than the original and built using stereolithography (SLA) fabrication methods. The MRV technique was used to measure the three-dimensional, three-component velocity field for a test case with a Reynolds number range of 2,600 to 7,600 based on diameter of the impingement holes. Flow distribution, impingement hole performance, and cross flow effects are discussed for the experiment in which a dilute aqueous copper sulfate solution was used as the working fluid. A paired experiment with a geometrically similar design employed electrical heating of a thin stainless steel shim to model a constant heat flux boundary condition of an interior wall of the turbine vane. The modeled vane insert was then operated with air as the working fluid at two test conditions with Reynolds numbers in the range of approximately 1,200 to 7,600 based on the diameter of the impingement holes. An IR camera was used to measure the surface temperature of the shim. Using energy balances and the known heat flux, the temperature data were used to determine heat transfer characteristics of the impinging jets for the pressure and suction side surfaces, including the Nusselt number. The MRV and IR data sets provide detailed insight into the surface effects of the flow distribution and the result on the local and area-averaged heat transfer performance. A strong coupling between the velocity field and temperature data provide insight into design feature performance, and serve as a validation data set for matched computational simulations. Finally, a comparison with internal heat transfer correlations is presented using the data from Florschuetz et al. [1]. The results showed a lack of agreement with Florschuetz, leading to the development of a novel methodology for estimating the heat transfer performance of an impinging hole with crossflow. Measurement uncertainty was estimated to be ±5% for velocity and ±5% for the spatially averaged Nusselt number distributions.

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.