Local Heat Transfer Characteristics Research Articles

Numerical simulation and experimental research of multi-pipe jet fireball

This paper studies a self-designed high temperature fireball quasi-static simulation device to understand the temperature distribution of each substance inside the fireball under the multi-pipe injection technology. The changes in the physical field of the high temperature fireball are obtained through numerical simulation. The simulation model in this paper adopts the turbulent k-? model, and simulates the material transfer process according to Fick's law. The commercial software COMSOL multiphysics is used to give and analyze the local characteristics of fluid-flow and heat transfer. The simulation device formed a high temperature fireball under the interaction of the three-way jet fire. The experimental data verified the accuracy of the simulation results, and the experimental results were in good agreement with the simulation results. This method of analyzing the actual temperature field through simulation can be used to provide a powerful verification scheme for the detection of complex multi-temperature fields in the future.

Local instantaneous heat flux measurements in an internal combustion engine using a MEMS sensor

• Local instantaneous heat flux was clearly measured using a resistance-type sensor. • Local instantaneous heat flux exhibited a strong cycle-to-cycle variation. • Influence of turbulent eddy on heat flux was detected experimentally. • Local heat transfer coefficient also exhibited a strong cycle-to-cycle variation. Wall heat transfer is one of the most important aspects of internal combustion engines. In this study, the local instantaneous heat transfer characteristics were investigated using an originally developed thin-film resistance-type sensor. The sensor, which was fabricated by micro-electro-mechanical systems (MEMS) technology, had advantages in the sensitivity, the spatial resolution, and the number of measurement points compared with conventional sensors. The MEMS sensor could clearly detect the cycle-by-cycle heat flux. Consequently, it was found that the local instantaneous heat flux had a strong cycle-to-cycle variability, the magnitude of which was in the order of 0.1–1 MW/m 2 , not only in fired operations but also a motored operation. The heat transfer coefficients also exhibited a strong cyclic variability with a magnitude of 0.1–1 kW/(m 2 ⋅K). These results indicated that the heat transfer characteristics could not be properly evaluated only by the conventional analysis based on the ensemble-averaged data. Additionally, it was found that the heat fluxes at the adjacent points exhibited similar but different values, indicating that there were turbulent eddies of sub-millimeter scale and they affected the local heat transfer. Since the local instantaneous heat transfer had quite different characteristics from the average, advanced design and control based on the understandings of the local instantaneous characteristics will be need for the further improvement of the engine’s thermal efficiency.

An experimental approach to determine local heat transfer characteristics in additively manufactured cooling configurations

Transient heat transfer measurements are performed on an additively manufactured real-scale cooling configuration with ribs on one side. A test rig is presented that operates at elevated pressure and provides a sudden temperature step in the fluid to heat the model walls. It enables the establishment of application-relevant flow conditions under which the local heat transfer characteristics are investigated while the manufacturing-related wall roughness can be preserved. The small dimensions prevent a direct temperature measurement in the cooling channels and a direct observation of the heat transfer surface is not possible. Thus, the fluid temperature is measured upstream and downstream of the model and the temperature response on the outer surface is measured by infrared thermography. Analytical modeling of the fluid temperature and a numerical method for calculating the 3D temperature propagation inside the model wall enable the determination of local heat transfer coefficients on the inner heat transfer surface. The crucial temperature data filtering and its effect on the results is described. A sensitivity analysis is performed to determine the best-suited evaluation times locally. The results of the developed method are compared to results from former measurements applying a well-established evaluation method on an up-scaled model with similar geometry.

Thermal performance of a central-jetting microchannel heat sink designed for a high-power laser crystal

Thermal management has become one of the critical problems in high-power laser systems. The laser crystal, which dissipates a heat flux over 150 W/cm2, is the most challenging component when solving the thermal problem. In this work, a specialized central-jetting microchannel heat sink (MCHS) is proposed purposely for a small-size laser crystal with high heat fluxes. Considering the space constraint and installation restriction, the MCHS has a narrow width to fit with the laser crystal, while its length is extended to enhance the heat dissipation capability. The MCHS is integrated by welding the two layers completely through a vacuum brazing method. Central-jetting impingement is introduced to enhance the cooling efficiency. Coolant is flushed vertically into the microchannel layer toward the central heating region where the heating source is attached at the central bottom of the MCHS. We conduct a parametric study on the thermal performance of the MCHS by experiments. The overall heat transfer coefficient of the MCHS achieves 50 × 103 W/(m2•K), and the average heat transfer coefficient of the microchannel reaches 21.5 × 103 W/(m2•K). The surface temperature of the heating head is controlled below 65 °C, and the temperature variation is less than 5 °C. In the solid bottom of the MCHS, thermal resistance in vertical direction is over 1.8 times of that in the horizontal direction, ensuring an efficient heat dissipation in the extended volume. The local heat transfer characteristics are further examined through additional numerical simulations. The wall temperature of the microchannel and the local heat transfer coefficient have a high plateau in the heating region, and then drop significantly along the flow direction. Due to heat transfer enhancement by jet impingement, the heat transfer coefficient in the central region is generally 15% higher than that averaged on the full microchannel.

The study of spray impact on the heated sapphire plate

The process of the liquid spray impact on the heated surface is studied experimentally using the IR-transparent sapphire plate method. The spatiotemporal distribution of the temperature field on the sapphire substrate surface during impacting spray is received. The obtained experimental data are an important step in a study of the local characteristics of heat transfer in the areas of the contact lines during liquid spray impact on the heated surface. Further, the local heat fluxes and heat transfer coefficients will be determined by solving the problem of thermal conductivity in the sapphire substrate.

Experimental study of the heat transfer of two parallel impinging jets

Local and integral characteristics of heat transfer are obtained at varying the Reynolds number Re = 5500, 11000, the distance between the jets y/D = 1.8, and the distance from the jets to the surface z/D = 0.5-10 for the system of two identical impinging jets. It is found in experiments that the effect of an adjacent jet leads to enhancement of local heat transfer at large distances between the nozzles and the barrier. It is also shown that an increase in the Re number increases integral heat transfer, and, at the same time, weakens the inter-jet interaction. The paper analyzes the scenarios of the behavior of local and integral heat transfer depending on the geometric and flow parameters of the system of two circular turbulent jets.

Influence of synthetic air jet temperature on local heat transfer characteristics of synthetic air jet impingement

The influence of synthetic air jet temperature on local heat transfer distribution of a synthetic air jet impinging on a smooth flat surface is investigated experimentally. The actuator frequency (f = 100 Hz to 300 Hz) and jet to orifice spacing (z/d = 1 to 8) is considered for the present study. The local heat transfer study is obtained using thin foil and IR thermal imaging technique. At z/d = 1, the Nusselt number characteristics are found approximately 2 times greater when synthetic air jet temperature is considered as jet air temperature instead of surrounding air temperature for all the frequencies. The experimental results observed that heat transfer distribution is strongly influenced by synthetic air jet temperature especially at lower orifice to jet distance.

Numerical analysis of effects of fins and conductive walls on heat transfer in side heated cavities — Onset of three-dimensional phenomena in natural convection

In the present study, we analyse individual and combined effects of conductive horizontal walls and conductive fins on the natural convection of air in side heated cavities (SHC). The flow and heat transfer are studied for Rayleigh numbers in the range of 104−109: Direct Numerical Simulation (DNS) is conducted for the lower and Large Eddy Simulation (LES) for the higher Rayleigh numbers (>108). Thermally conductive walls destabilize the flow yielding an earlier transition to turbulence and expedite the decay in boundary layer thickness with increase in Rayleigh number. The preheating/precooling along the conductive walls reduces the actual heat transfer at the vertical walls. Above the fin, instabilities are only marginally enhanced for adiabatic horizontal walls, whereas for conductive horizontal walls, plumes erupt from the fin. This localized Rayleigh-Bénard-like effect triggers 3D instabilities in the entire flow field and yields a steeper slope in Nusselt-Rayleigh diagram. The presence of a fin increases the integral heat transfer by 18% for adiabatic and 21% for conductive horizontal walls. We show that 2D and 3D simulations are similar for the smooth cases (i.e., without fin), but differ by 4% and 13% for the adiabatic and conductive fin cases respectively. The local heat transfer characteristics even deviates up to 50%, therefore a 2D simplification should be avoided.

Experimental investigation of unsteady convective heat transfer under airflow velocity and temperature variations

An experimental approach to study unsteady local heat transfer characteristics due to airflow velocity and/or airflow temperature variations is presented. It uses controlled electrical heaters and rotating vanes to independently vary the flow and thermal boundary conditions. Time-resolved surface temperatures are measured using an in situ calibrated infrared thermography camera. Those surface temperatures are analyzed by modeling the transient conjugate heat transfer process in the wall to obtain locally resolved surface heat flux distributions. The applicability is illustrated for a flow and heat transfer behind a tetrahedral vortex generator on a flat plate.

Local & Average Heat Transfer and Friction Factor Characteristics of a Flow through a Helical Wire Coil for Turbulent Flow Conditions

Heat transfer augmentation is essential for numerous industrial applications. The flow through a helical coil is one of the most efficient and effective techniques of heat transfer enhancement due to the provision of a large surface area. The present study is focused on the thermo-hydraulic performance of turbulent flow through a helical coil, made up of copper with a curvature ratio of 0.0519 (d/D) and pitch equal to 50 mm using water as a test fluid. The objective of the present study is to reveal the influence of secondary flow, centrifugal force, and Reynolds number (Re) on local velocity, local heat transfer coefficient in a helical coil. The values of average heat transfer and friction factor at different Re for a flow through a helical coil are compared with corresponding values for a flow through a smooth tube. The numerical results are presented for the Re number range at inlet conditions of the helical coil from 5000 to 20000 in the increment of 2500 under constant heat flux conditions. Numerical results indicate that there is a circumferential variation of local velocity and local Nusselt number (Nu) values concerningthe inner and outer sides of the helical coil. The values of average heat transfer are observed to be 150 to 200% more than the smooth tube values with increased pressure drop.

Numerical investigation on thermal hydraulic performance of hybrid wavy channels in a supercritical CO2 precooler

Heat transfer enhancement within the precooler in supercritical CO2 Brayton cycles is challenging due to the dramatic variations of thermophysical properties. The local flow and heat transfer characteristics of a straight channel PCHE are analyzed firstly in this paper and the idea of thermophysical property-matching partitioned heat transfer enhancement is proposed accordingly. Subsequently, three types of hybrid channel, where sinusoidal wavy structures are combined with straight channel in different ways, are designed to examine the effectiveness of the partitioned heat transfer enhancement idea. A comparative study on the thermal hydraulic performance is performed with different geometrical parameters under different working conditions. The results indicate that the partitioned heat transfer enhancement shows great advantages in reducing the total pressure drop under the given heat flux boundary conditions. Compared to the full-wavy and the other two hybrid channels, the overall thermal hydraulic performance of the type-C channel is the best. A maximum pressure drop reduction of 23% could be observed in the type-C channel (with the wavy section used in the low-temperature region) compared to that in the type-A channel (with the wavy section used in the high-temperature region). Such advantage will become more prominent when the outlet temperature of the precooler is closer to the pseudocritical temperature. Moreover, the averaged increment of local heat transfer coefficient in the wavy section is 1.56 kW/m2⋅K for the type-C channel when compared to the straight channel section. It's also found that the effects of gravity on the local heat transfer coefficient are almost negligible in the high temperature region, while heat transfer in the low temperature region can be affected by changing the placing direction of the precooler. The present study may provide a practical guidance on the development of highly efficient heat exchangers where the thermophysical properties of the working fluid are highly variable.

Investigations of Flow and Heat Transfer Characteristics in a Channel Impingement Cooling Configuration with a Single Row of Water Jets

The present study experimentally and numerically investigates the effect of channel height on the flow and heat transfer characteristics of a channel impingement cooling configuration for various jet Reynolds numbers in the range of 2000–8600. A single array consisting of eleven jets with 0.8 mm diameter injects water into the channel with 2 mm width at four different channel heights (3, 4, 5, and 6 mm). The average heat transfer coefficients at the target surface are measured by maintaining a temperature difference between the jet exit and the target surface in the range of 15–17 °C for each channel height. The experimental results show the average heat transfer coefficient at the target surface increases with the jet Reynolds number and decreases with the channel height. An average Nusselt number correlation is developed based on 85 experimentally measured data points with a mean absolute error of less than 4.31%. The numerical simulation accurately predicts the overall heat transfer rate within 10% error. The numerical results are analyzed to investigate the flow structure and its effect on the local heat transfer characteristics. The present study advances the primary understanding of the flow and heat transfer characteristics of the channel impingement cooling configuration with liquid jets.

Natural Convection Melting Influence on the Thermal Resistance of a Brick Partially Filled with Phase Change Material

The constant growth of urban agglomerations with the development of transport networks requires the optimal use of energy and new ways of storing it. Energy efficiency is becoming one of the main challenges of modern engineering. The use of phase change materials in construction expands the possibilities of accumulating and storing solar energy, as well as reducing energy consumption. In this study, we consider the problem of the effect of natural convection on heat transfer in a building block containing a phase change material. Heat transfer, taking into account melting in brick, was analyzed at various temperature differences. The mathematical model was formulated in the form of time-dependent equations of conjugate natural convection using non-dimensional stream function, vorticity, and temperature. The equations describing melting, taking into account natural convection, were solved using the finite difference method. Smoothing parameters were used to describe phase transitions in the material. As a result of calculations, local characteristics of heat and mass transfer at various points in time were obtained, as well as changes in temperature profiles on the side surfaces. It is shown that with a large volume of melt, natural convection increases heat loss by more than 10%.

On heat transfer and flow characteristics of jets impingement on a concave surface with varying pin-fin arrangements

Numerical simulations have been conducted to investigate the effects of pin-fin arrangement and jet Reynolds number on heat transfer enhancement and flow characteristics of a concave confined jet impingement cooled surface conjugated with pin-fins. The results without a pin-fin are used as a baseline. Vortices and different sections of streamlines in the channel are employed to understand the flow structures in this cooling system. Based on this, topology pictures are obtained with skin-friction lines to discuss the fluid flow near the target and pin-fin surface. The local and averaged heat transfer characteristics are analyzed using the flow characteristics. The overall parameters (friction factor, thermal performance, pumping power) are also obtained. The results show that the appearance of the pin-fin plays a positive role in heat transfer enhancement and uniformity. The case of a 35° pin-fin arrangement angle reaches the highest overall Nusselt number and heat transfer uniformity. The far away movement of the pin-fin is beneficial to decrease pumping power at a constant heat transfer. The thermal performance is highest for 55° pin-fin arrangement angle at a jet Reynolds number of 40,000 in this work.

Local heat transfer characteristics of synthetic air jet impinging on a smooth convex surface

The local heat transfer characteristic of synthetic air jet impinging on convex surface is studied experimentally. The heat flux on the convex surface is maintained constant. The experiment is carried out for different actuator frequency (f = 100 Hz–300 Hz), jet to plate distance (z/d = 1 to 8) keeping the orifice diameter constant. The local heat transfer characteristic of synthetic air jet on the convex surface is obtained by IR thermal imager using thin foil technique. Local heat transfer characteristics is studied in terms of local Nusselt number along the axial (x/d at r = 0) and radial direction (r/d at x = 0) of convex surface. The obtained result shows that the local heat transfer is strongly influenced by actuator frequency and jet to plate distance. The actuator frequency (f = 200 Hz) and lower jet to plate distance (z/d = 2) is observed effective at all jet to plate distance and actuator frequency. The correlation of maximum local Nusselt number in terms of Reynolds number is obtained with the maximum deviation of 18%. The maximum uncertainty in measurement of average Nusselt number is observed less than ±6%.

Effects of stagnating and thermal shielding of an upstream promoter on forced convection of flow past a square cylinder in a channel

This work performs a direct simulation on forced convection heat transfer of flow past a square cylinder in a channel with an upstream promoter. The promoter is small in size that the incoming flow is stagnated and heated, thus the flow in the gap between the promoter and main cylinder keeps steady and the heating by the main cylinder gets weak as it develops downstream. The objective of the present study is to explore the effects of two parameters, i.e., Reynolds number and gap ratio, on the thermal and flow characteristics for Re = 70-140 and GR = 2-8. The effects of the two parameters are presented and analyzed by the thermal and flow fields, variations of global characteristics quantities, local heat transfer characteristics, patterns of flow in the gap and near-wake region, and temperature on the channel walls. We emphasize both time-averaged and fluctuating quantities to reflect the mean and pulsating characteristics of the flow. Numerical results reveal that although the promoter produces a steady flow in the gap, it alters the velocity profile of the gap flow and consequently changes the shearing properties, thus influences the flow around and even in the wake downstream of the main cylinder. In general, the small promoter partially stabilizes the wake flow and reduces the amplitude of fluctuating forces acting on the main cylinder compared with the single cylinder configuration, and the stabilization is more remarkable for small gap ratios. However, the fluctuation of heat transfer rate on the surface of main cylinder may increase. A further analysis indicates that as the promoter is placed close to the main cylinder, the recovery rate of deficit wake flow is slower and the fluctuating amplitude of velocity and temperature is relatively smaller due to the stabilized flow.

Experimental investigation on thermal performance of a pumped two‐phase battery cooling system using mini‐channel cold plate

This study presents an experimental investigation on a pumped two-phase battery cooling system using R1233zd(E) as the refrigerant. The thermodynamic cycle of the system, together with the heat transfer and pressure drop characteristics of the mini-channel cold plate, were analyzed under different conditions. Results showed that the pressure rise of the pump was increased under higher heat flux and mass flux, and the overall system pressure was elevated when the coolant bath temperature was raised. The effects of the mass flux on the overall heat transfer coefficient of the cold plate were different under various heat fluxes, while pressure loss of the cold plate showed a dramatic rise after the heat flux was over 2 W·cm−2. The average temperature of the cold plate was generally decreased with the increasing mass flux, while the maximum temperature difference was nearly constant when the heat transfer was dominated by single-phase forced convection. However, the maximum temperature difference could be lowered after flow boiling became dominant. In addition, the local heat transfer characteristics for different channels and the temperature uniformity in the cold plate were investigated. It was found that the local temperature was increased and then stabilized along the flow direction, while the local heat transfer coefficient was increased monotonously. Meanwhile, lower local temperature and higher heat transfer coefficient were found in the upper channel for the intensive boiling with lower mass flux which compared to the middle channel. Novelty Statement This study investigated the performance of a pumped two-phase battery cooling system using mini-channel plate under higher heat flux and discussed the reasons for wall temperature variation and temperature uniformity under different heat transfer mechanisms.

Buoyancy effects on convective heat transfer of supercritical CO2 and thermal stress in parabolic trough receivers under non-uniform solar flux distribution

The buoyancy influenced heat transfer characteristics of S-CO2 have been investigated widely in previous studies, but limited to the conditions with constant wall temperature or constant heat flux. The present study numerically investigates the buoyancy effects on the convective heat transfer of S-CO2 and the thermal stress in parabolic trough receivers (PTRs) under the non-uniform solar flux distribution based on a coupled optical-thermal-fluid-mechanical numerical model. The results indicate that the drastic density variation associated with the large temperature gradient of the fluid occurs on the cross-section under the non-uniform solar flux distribution, which further induces the secondary flow. As the secondary flow improves the synergy between the velocity vector and the temperature gradient, the bulk average heat transfer is prominently enhanced due to the buoyancy effect. However, the buoyancy effects on the local heat transfer characteristics are more complex. The buoyancy effect enhances the heat transfer at the bottom but impairs the heat transfer at the top, and the heat transfer enhancement area is expanded along the flow direction. The temperature distribution in the absorber tube wall is greatly flattened due to the buoyancy effect, and the thermal stress is consequently reduced. The buoyancy effects on the thermal and mechanical performances become less significant with the increase in the operating pressure, the inlet velocity and the inlet temperature but with the decrease in DNI. The buoyancy effect in the PTR under non-uniform solar flux distribution can be ignored when Se/Re<0.3, Ri<0.04, Bup<0.002, Buj<80, or Grq/Grth<70.

Combined experimental and numerical studies on flow characteristic and heat transfer in ribbed channels with vortex generators of various types and arrangements

This work numerically and experimentally studies the heat transfer performances and flow characteristics of vortex generators (VGs) in a high aspect ratio rectangular ribbed channel with the Reynolds number ranging from 20000 to 100000. The steady liquid crystal thermography (LCT) method is adopted to link the images captured by experiments and the Nusselt numbers. Four different cases combining ribs and VGs are considered, and they are compared with a case with five continuous ribs in the channel. For the first time, the effect of winglet VGs and the tetrahedral VGs on heat transfer performance are compared. Simultaneously, two different arrangements of VGs, i.e. common-flow-down VGs and common-flow-up VGs are also studied to figure out which arrangement is better for improving the heat transfer performance. Simulations are conducted to systematically compare the fluid flow and local heat transfer characteristics to reveal the underlying thermo-fluid mechanisms of the considered cases by using three-dimensional CFD numerical simulations with a verified turbulence model, i.e., the SST k-ω model in the transition state. Results indicate that in the range of 0 < x/P < 1, the Nusselt number is enhanced to a large extent due to larger space for the flow reattachment. In addition, it is found that the winglet VGs perform better than the tetrahedral VGs. The winglet VGs with the common-flow-down arrangement give the best overall thermal performance at all Reynolds numbers in most cases. More specifically, the heat transfer performance can be improved up to 5.51% by adopting the common-flow-down VGs compared with the case having five continuous ribs. Furthermore, the flow characteristics reveal that longitudinal VGs can generate vortices to disturb the boundary layers, which can enhance the flow mixing and augment the heat transfer performance.

Study on natural convection heat transfer in a closed cavity with hot and cold tubes.

Laminar natural convection with a pair of hot and cold tube in a closed cubic cavity is carried out. This configuration can be founded in performance of nuclear power plant containment passive residual heat removal system. The basic government aquations are sloved by means of finite volume method. The effect of Ra number (103-106), shape of tube and spatial position on local and mean heat transfer characteristics is studied. It is found that the Nu number increased when raising Ra number. The Nu number is higher when the shapes are circle and triangle. In adddition, it is founded that the heat transfer has a better effect when the cold tube locates above the hot tube among the five spatial positions. The results provides theoretical basis for performance of nuclear power plant containment passive residual heat removal system.