Flow Heat Transfer Performance Research Articles

Experimental study on flow boiling heat transfer in rigid-tail rib-channel under rolling motion conditions

This study proposes a novel addition of rigid filamentous tail structure at the trailing edge of the ribs to enhance the heat transfer performance of flow boiling in mini-channel under rolling conditions. The flow boiling heat transfer characteristics of Novec649 in both rib-channel and rigid-tail rib-channel are experimentally investigated. Heat flux, inlet flow velocity, rolling angle and rolling period are considered as the variable parameters, which are 50 – 300 kW·m−2, 0.3 – 0.9 m·s−1, 0 – 15°, 10 – 20 s, respectively. The experimental results show that the rigid-tail structure improves the heat transfer performance of the channels within the entire range of thermal parameters, while increasing the pressure drop at high heat flux and/or high velocity. Compared with rib-channel, the rigid-tail structure exhibits the strongest enhancement in flow boiling heat transfer performance under high heat flux of qeff = 300 kW·m−2 and high flow velocity of vin = 0.9 m·s−1, and the heat transfer coefficient can be increased by 6.93 % at most. And increasing the rolling angle or decreasing the rolling period will decrease the average wall temperature by a maximum of 1.94 °C and increase the heat transfer performance factor j by a maximum of 8 % at vin = 0.3 m·s−1, qeff = 200 – 300 kW·m−2 for the rigid-tail rib-channel. Furthermore, the addition of a rigid-tail structure helps maintain smaller bubble sizes and a more uniform distribution in the mainstream region of the two-phase flow.

Enhancement of Flow Boiling Heat Transfer Characteristics on Diamond/Cu Heterogeneous Surface

Surface treatment is an effective method of enhancing the heat transfer performance of flow boiling. Diamond/Cu, as a particle‐reinforced composite, exhibits heterogeneous surface properties and surface microstructures that play distinct roles in heat transfer. To elucidate the synergistic effect of heterogeneous material surfaces and microstructures, pure copper surface (PC), untreated diamond/Cu surface (DC), and surface‐treated diamond/Cu surface (SDC) are prepared for this experiment. These surfaces demonstrate various combinations of diamond, copper, and their corresponding surface microstructures. DC, characterized by heterogeneous wetting properties, demonstrates a heat transfer coefficient (HTC) that is 9%–24% higher than that of PC. The surface treatment of SDC results in the formation of a superhydrophilic CuO nanosheet structure, significantly enhancing both surface roughness and energy. SDC develops microcracks on the surface of diamond. The increased roughness provides numerous nucleation areas for the hydrophobic diamond, enhancing the overall wettability of the surface when combined with the superhydrophilic CuO region. SDC demonstrates the advantage of enhanced flow boiling on the surface of heterogeneous materials. At the identical heat flux, SDC exhibits a reduction of 5–10 °C in wall superheat and an increase of 38%–47% in HTC.

Predictive Modeling for Microchannel Flow Boiling Heat Transfer under the Dual Effect of Gravity and Surface Modification

This paper investigates the heat transfer performance of flow boiling in microchannels under the dual effect of gravity and surface modification through both experimental studies and mechanistic analysis. Utilizing a test bench with microchannels featuring surfaces of varying wettability levels and adjustable flow directions, multiple experiments on R134-a flow boiling heat transfer under the effects of gravity and surface modification were conducted, resulting in 1220 sets of experimental data. The mass flux ranged from 735 kg/m2s to 1271 kg/m2s, and the heating heat flux density ranged from 9 × 103 W/m2 to 46 × 103 W/m2. The experimental results revealed the differences in the influence of different gravity and surface modification conditions on heat transfer performance. It was found that the heat transfer performance of super-hydrophilic surfaces in horizontal flow is optimal and more stable heat transfer performance is observed when gravity is aligned with the flow direction. And the impact of gravity and surface modification on heat transfer has been explained through mechanistic analysis. Therefore, two new dimensionless numbers, Fa and Conew, were introduced to characterize the dual effects of gravity and surface modification on heat transfer. A new heat transfer model was developed based on these effects, and the prediction error of the heat transfer coefficient was reduced by 12–15% compared to existing models, significantly improving the prediction accuracy and expanding its application scope. The applicability and accuracy of the new model were also validated with other experimental data.

Numerical investigation on flow and heat transfer characteristics in honeycomb-like microchannel heat sink encapsulated with phase change material

The present study proposes a novel model to numerically investigate the flow and heat transfer characteristics of a honeycomb-like microchannel heat sink (MCHS) encapsulated with phase change material (PCM). Compared with a single MCHS, the hybrid MCHS combines the advantages of microchannels in terms of micro efficiency with the isothermal phase change effect of PCM. The finite volume method is used to discretize the three-dimensional flow and heat transfer processes within the hybrid MCHS. Special attentions are paid to the effects of PCM, geometric parameters and local hot spots on the temperature uniformity and flow heat transfer performance. The results show that the honeycomb inflow structure design and the thermal management characteristics of PCM phase change cooling exhibit better temperature uniformity and cooling performance. The overall performance of the heat sink is best at microchannel height of 0.09 mm and inlet number of 5 under the range of parameters studied. Adjusting the channel height from 0.06 mm to 0.09 mm at a volume flow rate of 10 mL∙min−1 can reduce the thermal resistance by 51 % and the pumping power by nearly 20 %. Increasing the number of microchannel inlets results in better heat transfer performance, but this improvement comes at the expense of larger pressure drop. When a local hotspot occurs, the hybrid MCHS can reduce the temperature peak by 2–5 K, achieving a smoother temperature distribution.

Numerical Investigation on Two-Phase Flow Heat Transfer Performance and Instability with Discrete Heat Sources in Parallel Channels

With the rapid development of integrated circuit technology, the heat flux of electronic chips has been sharply improved. Therefore, heat dissipation becomes the key technology for the safety and reliability of the electronic equipment. In addition, the electronic chips are distributed discretely and used periodically in most applications. Based these problems, the characteristics of the heat transfer performance of flow boiling in parallel channels with discrete heat source distribution are investigated by a VOF model. Meanwhile, the two-phase flow instability in parallel channels with discrete heat source distribution is analyzed based on a one-dimensional homogeneous model. The results indicate that the two-phase flow pattern in discrete heat source distribution is more complicated than that in continuous heat source distribution. It is necessary to optimize the relative position of the discrete heat sources, which will affect the heat transfer performance. In addition, compared with the continuous heat source, the flow stability of discrete heat sources is better with higher and lower inlet subcooling. With a constant sum of heating power, the greater the heating power near the outlet, the better the flow stability.

Effect of Aspect Ratio and Installation Direction on Channel Flow Heat Transfer Performance under a Wide Range of Reynolds Number

In recent years, in the cooling technology for high-power electronic devices such as power transistors used for drive motor control of electric vehicles and hybrid vehicles, a method of flowing a cooling fluid to a cooling substrate having a fin structure has become the main technology. The structure of the cooling fluid flow path is a channel flow through multiple narrow plate gaps to secure a heat transfer area. In this study, the heat transfer characteristics when the aspect ratio of the channel having a flat rectangular cross-section was changed were investigated in detail by experiments. Moreover, the difference in the heat transfer characteristic at the time of making a rectangular flow path into vertical installation and horizontal installation was also investigated.

Confined bubble growth and heat transfer characteristics during flow boiling in microchannel

Bubble behaviors are closely related to the heat transfer performance during flow boiling in microchannel, however, the effect of channel cross-section decreasing on the bubble growth is still not fully understood at present. In this work, an experimental investigation is conducted to investigate the bubble growth characteristics during flow boiling in a single microchannel with 0.5mm×1mm rectangular cross-section, and the heat transfer performance of flow boiling and its influencing factors are studied. Experiments are conducted with subcooled deionized water and the bubble behaviors are visualized by a high speed CCD camera installed upon the test section. Depending on the heat flux, different growth features are observed in the bubble growth process. Two kinds of bubble growth model are identified: the power law model in initial growth period and the linear law model in later period. The confinement effect of the microchannel is deemed as the mechanism causing the alteration of bubble growth models during its growth process. The deformation features of confined bubble are discussed to illustrate the intensification of evaporation on the liquid–vapor (LV) interface at bubble root, which increases the growth rate of bubble in its confined growth period as well as the heat transfer capability of bubble. Therefore, the maximum local heat transfer coefficient along the channel is found in the region where confined bubble and/or short elongated bubble flow pattern are dominant. Moreover, the heat flux is found to have great influence on the overall heat transfer performance of flow boiling in microchannel, but the effect of mass flux is much less.

Effect of pin fin to channel height ratio and pin fin geometry on heat transfer performance for flow in rectangular channels

The efficiency of a thermoelectric generator (TEG) can be defined as the ratio of the power output to the heat input at the hot side of the device. This ratio is governed by the laws of thermodynamics and thus cannot exceed the Carnot efficiency. It follows that the greater the difference between the temperatures of the hot and cold sides of the device, the greater the efficiency and power output from the TEG. This study focuses on effective techniques to enhance heat transfer on the hot side of the TEG in order to increase the total power output from the device. In this study heat transfer enhancement mechanisms are evaluated for the hot side of a TEG system generating power from waste heat in automobile exhaust gases. The use of pin fins was examined, as they are a common and effective way to increase heat transfer in a channel. Heat transfer enhancement measurements are presented with 3-dimensional partial pin fin arrays of circular, triangular, hexagonal, and diamond shapes on the walls of a rectangular channel representing the hot side of the TEG system and the automobile exhaust duct. Channel heights are varied to measure the effect of the pin fin height to channel height ratio while keeping the pin fin height constant. Channel hydraulic diameter and configuration of the fins were chosen based on existing literature. Pin fin performance is studied over a range of Reynolds numbers, calculated based on full channel height. The pin fins with the best initial performance have been further analyzed by varying the channel height in order to change the pin fin to channel height ratio while keeping the hydraulic diameter to pin fin height ratio constant. The experiments use the transient liquid crystal method to measure detailed heat transfer coefficients on the test surface. Results show that the diamond pin fins perform the best in enhancing heat transfer. Lower channel heights that cause pin fins to block 50% of the channel provide significantly higher heat transfer coefficients.

Optimal surface fractal dimension for heat and fluid flow in microchannels

The fractal Weierstrass–Mandelbrot function was introduced to characterize the multiscale self-affine rough surface of microchannels. Based on this fractal characterization, the role of the rough surface structure on the thermal and hydrodynamic properties in microchannels was evaluated using a computational fluid dynamic simulation. Once identified, these were used to determine the optimal surface dimension for heat and fluid flow. It was found that, no matter what the Reynolds number and roughness height are, the flow heat transfer performance is being optimized with increasing fractal dimension of the surface until to the dimension value of three (infinitely crumpled).