Temperature non-uniformity on chips has drawn the attention of researchers due to unwanted thermal stress development on chips resulting in a reduction in their life cycle and performance. In the present investigation, the heat transfer and flow characteristics of the coolant in the micro-pin-fin heat sink with variable density arrangement have been conducted numerically. The dimensions of the micro-pin-fin heat sink are 18.0 mm × 19.0 mm × 4.0 mm, and the height of the micro-pin-fin is 2 mm. Circular micro-pin-fin with diameters of 400, 500, and 600 μm, respectively have been considered. The power supply is 50 W with a heat source area of 10.0 mm × 10.0 mm. Water was the working fluid used while aluminum was used for the solid part of the heat sink. The operating pressure drop between the inlet and the outlet of the heat sink is fixed at 1500 Pa, 3000 Pa, and 5000 Pa. Ansys-Fluent was employed for the analysis. The results indicate that the temperature uniformity on the heat source for heat sink with variable density arrangement is better than that with staggered arrangement for about the same number of micro-pin-fins. The best effective thermal resistance is noted as 0.258 K/W among the heat sinks with all the configurations. In addition, the temperature difference per unit length on the heat source for heat sink with convergent arrangement at the pressure difference of 5000Pa and micro-pin-fin diameter of 600 μm was 1.34 K/mm, which is lower than the previously reported literature.

This paper explores the design parameters of high-voltage coils of three-phase transformers and the effects of eddy current losses caused by energy conversion. The commercial software, ANSYS-Maxwell, was employed to conduct the simulation of electrical and magnetic fields. The design parameters of the high voltage coil include the leg distance of core, the height and the block thickness of primary windings, the height of primary coils, and the height of secondary windings. The specification of the three-phase transformer of this study are 3000 kVA with rated voltage of 6600 V and corresponding current of 151.5 A. Base on thirty cases of simulation, machine learning, artificial neural network, was utilized to predict the extra loss due to eddy current in the clamps and the windings. The prediction accuracies are 0.72 and 0.86 for primary and secondary windings, respectively.

Methanol is regarded as an important feedstock for hydrogen production due to its high energy density and superior transportability. A tubular packed-bed reactor performing the methanol steam reforming (MSR) process was modeled by adopting computational fluid dynamics (CFD) software to analyze its performance. Kinetic parameters of the reactions were adjusted according to the literatures and our previous experimental results. The methanol conversion, the hydrogen production rate, and the CO concentration in the produced mixture were evaluated by considering different levels of the length and temperature of the catalyst bed, the steam-to-carbon ratio and the space velocity of the feedstocks. Moreover, the correlation between the dimensionless parameter, Damköhler number, and the methanol conversion was also investigated.

In this paper, the thermophysical properties and transient heat transfer characteristics of composite phase change materials (CPCMs) were investigated numerically and experimentally in details. Two kinds of micro-encapsulated phase change materials (MEPCMs) paraffin with melting temperatures of 28 and 37 degrees C, respectively, were employed in this paper. The CPCMs were fabricated by intercalating expanded graphite (EG) with average diameter of 45 m into MEPCMs. The addition amount of EG were 10%, 30%, and 50% in weight. The thermophysical properties, including density, thermal conductivity and specific heat, of the CPCMs were measured and the latent heat and specific heat were calculated from data with differential scanning calorimetry (DSC). In the experiment, the square enclosure has a cross-section dimension of 100 by 100 mm and it was 15 mm in thickness. The top side wall of the enclosure was heated with isothermal cooling on the bottom side wall while the remaining side walls were thermally insulated. The numerical model which is designed to meet the conditions of the experimental parameters was employed to examine the transient heat transfer characteristics for the CPCMs in the enclosure. Results show that MEPCMs with increasing EG would increase the thermal conductivity and the heat transfer rate. However, the specific heat of CPCMs would decrease. The numerical predictions agree well with the experimental data. In addition, the results disclose that the CPCMs could enhance the rate of heat transfer and energy storage, but minor loss in total energy storage.

In present manuscript, the flow field produced by oscillating foils in a duct were under investigation numerically. The flow field was considered as an incompressible laminar transient flow. In this paper, the oscillating frequencies were 2, 4 and 6 Hz; the span angles of the foil were 30˚, 60˚, 90˚, 120˚, and 150˚, respectively. The flow field produced by a particular oscillating pattern (fast forward slow backward, FFSB) has been made in comparison with that by the sinusoidal oscillating pattern. The flow field produced by dual oscillating foils was also analysed. Two foils oscillated in the same direction (in-phase) and in the opposite direction (counter-phase) were studied as well. Commercial software ANSYS-FLUENT was employed for the 2D numerical simulation. A moving grid technique was utilized in the analysis. Results present the flow average velocity increases as the oscillating frequency increases. It is also shown that the flow average velocity with span angle of 120˚ is the largest among these cases. In addition, the flow average velocity with dual foils is better than that with a single foil.

The cooling performance of a 30 kW switched reluctance motor (SRM) has been evaluated numerically to investigate the cooling design for this motor. The physical model of the 3D motor with exact size has been established using Solidworks. Both JMAG-Designer and ANSYS-FLUENT commercial software were employed to study the electromagnetics, velocity and temperature field for numerical analysis. Aluminum cooling fins were fitted on the cooling casing of the motor in order to enhance the convection heat transfer. Fin design with the same heat transfer area has been discussed. Both air cool and water cool designs of the 30 kW SRM have been examined and discussed for comparison as well. Results show that the maximum temperature in SRM cannot be reduced to a requested operating temperature with air cooling only even with fin design on the casing. They also indicate that liquid cooling is a better approach for SRM cooling. In addition, the Reynolds number of liquid coolant is the key parameter which affects the thermal cooling of SRM. In present study, the temperature rise of motor winding in SRM has been reduced to 42% with liquid water cooling and the maximum temperature in SRM is approximately 128.8 degrees C. (C) 2017 Elsevier Ltd. All rights reserved.

An electro-magnetic driven miniature piston pump with novel mechanism has been developed for testing the feasibility in the present study. The novel mechanism is a hollowed piston in axial direction with a membrane served as a check valve. The operation of this novel pump is driven by electro-magnetic force to produce reciprocating motion with periodical switch of magnetic poles. The detailed operating principle is described for the magnetic driven reciprocating pump. The working fluid is water in this investigation. Our results include the performance and characteristics of the novel miniature pump. Testing results show that the maximum head is linearly proportional to the applied electric current. In addition, the relation between the head and flow rate is also close to linear, except for extreme conditions. The hydraulic efficiency and the volume efficiency were calculated to be close as 0.96 and 0.95, respectively.

This paper experimentally investigates the heat transfer performance and characteristics of a heat sink with bio-mimetic oscillating foil. The oscillating foil is driven by a novel mechanism of grooved cam with linkage. The air flow is drawn into a rectangular duct through the motion of the bio-mimetic oscillating foil. Various heat sinks were set at the outlet of the duct to study the thermal performance. Two types of grooved cams were designed to obtain different angular velocity profiles for the oscillating foils. The angular velocity was specified as fast forward/slow backward and sinusoidal patterns. Foils with three flexural stiffness values were utilized in the study. The averaged velocity at the inlet was measured to estimate the Reynolds number and Strouhal number. The effects of configuration of heat sink design with oscillating foil were also under investigation. The junction temperature of the heat sink was measured to obtain the effective thermal resistance. Results show that the angular velocity profile and flexural stiffness of the oscillating foil have a significant effect on the thermal performance of the heat sink.

This paper examines the dynamic cell performance of a kW-grade proton exchange membrane fuel cell stack with anode dead-ended mode fuel supply. A self-made kW-grade 40 cells stack with reaction area of 112.85 cm(2) has been used in the experiment. A single-chip (DSPIC30F4011) is utilized for establishing a control circuit to monitor the voltage and current with constant-current loading. The stack temperature is controlled at a low-level temperature rise. To enhance the hydrogen utilization and reduce the water flooding in the fuel cell stack, the dead-ended anode operation is accomplished by controlling the open or close of the anode outlet solenoid valve. As the loading is heavy, the anode outlet solenoid valve is purged frequently to force the water to flow out. While a light load, the anode outlet solenoid valve is shut down for a period time for hydrogen saving. The solenoid valve is controlled to be opened, referred as purge interval, reaching the discharge amount for 1000 C, 1500 C, and 2000 C as parameter, respectively. The open period of solenoid valve, referred as purge duration, is set as 1 s, 3 s, and 5 s for this study. Experimental results indicate an optimal purge interval and duration for water management and cell performance of the fuel cell stack. (C) 2015 Elsevier Ltd. All rights reserved.

In this study, a microchannel heatsink with working fluid containing phase change material has been investigated numerically. The addition of phase change material in working fluid is to increase the heat transfer rate during cooling process. Commercial software ANSYS Fluent 15.0 is utilized to study the thermal resistance of the microchannel heatsink. Properties of working fluid with phase change material are with mass-averaged values. The numerical model for the phase change material used the latent heat as a function of heat capacity during phase change. This concept is modeled with user defined function in the simulation. The effects of microchannel geometry and the driven pressure difference between the entrance and exit of the heatsink on heat transfer performance are studied as well. The microchannel width varies in hundred micrometers with 2mm in height; the cross-sectional porosity of the fin array ranges from 0.6 to 0.7. Concentration of the phase change material in the working fluid is also a parameter in the investigation, ranging from 0% to 10%. The driven pressure difference of the working fluid is less than 30cm water head. The fin array covers an area of 1cm2, and the power supply is 60W, such that the heat flux is 0.6MW/m2.

Effects of film hole arrangement and geometry on impingement heat transfer along a film hole surface are experimentally investigated in detail. A transient liquid crystal thermograph technology has been used in the experiment for present investigation. The film hole size with four different values, 1.5, 2.0, 2.5, 3.0mm, jet Reynolds number ranging from 2000 to 4000, and jet-to-target spacing ranging from 1.5 to 4.5 are considered to study the impingement heat transfer performance. In addition, three arrangements of film hole on the target plates, named side-, middle- and staggered-types, are tested, respectively. The experimental results show that the Nusselt number increases with the increase of jet Reynolds number as well as the decrease in jet-to-plate spacing. Better heat transfer can be achieved with larger film hole size. As for the effect of the arrangement of film holes on the target surface, the heat transfer on side-type plate is more significant than the other two for smaller jet-to-plate spacing.

This paper numerically studies the heat transfer in heat sinks with micro-pin-fins. Commercial software, ICEPAK, was used to explore the effect of micro-pin-fin array design on the flow rate and heat transfer characteristics of liquid cooling heat sink. The parameters include the diameter of pin fin, the gap between the pin fins, and the relative position between the pin fins. The diameters of the pin fin are 0.5mm, 0.6mm, and 0.7mm, respectively. The gap between the pin fins are 0.2mm, 0.25mm, 0.3mm, 0.4mm, and 0.5mm, respectively. Three types of fin array arrangements are set with different longitudinal spacing and transverse spacing. The driving force of the working fluid is the pressure head with 500, 1500, and 3000 Pa, respectively. Results show that the effective thermal resistance ranges from 0.27 to 0.55 with the power density of 300kW/m2.

In this work, a two-phase three-dimensional numerical transport model based on the two-fluid method for the proton exchange membrane fuel cells (PEMFCs) with parallel flow field, interdigitated flow field, and serpentine flow field has been presented to study the cell performance and transport phenomena in the PEMFCs. The effects of width, height and aspect ratio of the flow channel on the cell performance and water removal with different flow fields are under investigation. Results show the liquid water removal increases as the channel height decreases, however, the cell performance decreases as well. It is also found that the cell performance decreases as the channel width increases in parallel flow field due to lower gas velocity with less water removal. The effect of channel aspect ratio approximate to 1 is particularly studied in this paper. It reveals that the cell performance is better as the channel cross-section area is smaller because of higher gas velocities.

This paper numerically and experimentally investigates the heat transfer performance and characteristics of liquid cooling heatsink containing microchannels. The effects of channel geometry and pressure drop between the entrance and exit of heatsink on the heat transfer performance are studied. The geometrical parameters include aspect ratio and cross-sectional porosity of the channels. The height of the microchannels is considered constant. The aspect ratio is set from 1.67 to 14.29 and the porosity is from 25% to 85%. The imposed pressure drop ranges between 490 and 2940 Pa. It is found that the aspect ratio corresponding to the lowest effective thermal resistance is changed with respect to the pressure drop. It is also noticed that the value of effective thermal resistance is almost a constant for cross-sectional porosity in the range of 53%–75%. The effective thermal resistance is increased when cross-sectional porosity is deviated from this range. In addition, the increasing of pressure drop enhances heat transfer performance for channels of high aspect ratio more than those of low aspect ratio.

This paper investigates the heat transfer performance and characteristics of liquid cooling heatsink containing microchannels. The porosity analysis is utilized to study the thermal resistance of the heatsink with microchannels. Both numerical simulation and experimental work have been carried out. Results show that the effective thermal resistance of the heatsink was affected by both channel aspect ration and the pressure gradient. It is found that channel aspect ratio of 8 present the lowest effective thermal resistance at low pressure gradients. However, the lowest effective thermal resistance appears for channel aspect ratio of 11 under high pressure gradients. As an overall inspection, the value of the effevtive thermal resistance ranges from 0.32 to 0.65°C/W with various porosity at low pressure gradients, while it ranges from 0.18 to 0.37°C /W at high pressure gradients.

In this work, the effects of jet geometry and the arrangement of film holes on the target plate on the impinging heat transfer are experimentally investigated in detail. A liquid crystal thermograph technology is employed in this study. The aspect ratios (AR) of elliptical jet with five different values, 4, 2, 1, 0.5, and 0.25, jet Reynolds number ranging from 2000 to 4000, and jet-to-target spacing ranging from 1.5 to 4.5 are considered to investigate impingement heat transfer performance. In addition, three arrangements of film hole on the target plates, named side-, middle- and staggered-types, are tested, respectively. The experimental results show that the Nu increases with the increase of jet Reynolds number. Better heat transfer is noted for the cases with smaller jet-to-plate spacing. For the effect of the arrangement of pores on the target surface, the heat transfer on middle-type plate is more significant than the other two for smaller jet-to-plate spacing. As for the effect of aspect ratio, results indicate that the optimal heat transfer performance is found with circular jet of AR = 1.

In this study, experiments were carried out to study the effects on the performances of individual cell and stack of PEM fuel cell. In the experiment, there are four key operating conditions that affect the cell performance, and they are gas humidification temperature, cell temperature, assembled torsion, and gas flow rate. A 5-cell stack of PEMFC was used to measure the voltage and current density for individual cell in this experiment. Results reveal that the performances of the center fuel cells are relatively lower than those of the cells on both sides of the stack. It is also shown that stack performance increases with the increase in the anode humidification temperature as well as the center cell of the stack. As for the effect of cell temperature, results indicate that stack performance increases with the increase in cell temperature. It is also disclosed that the performances of individual cell and stack do not change with the increase in the anode gas stoichiometric ratio, but increase with the increase in the cathode gas stoichiometric ratio. In addition, the experiment results also show that the whole stack's performance is enhanced with the increase in the assembling torsion.