Direct numerical simulation (DNS) is performed to solve the governing equations for fluid flow and heat transfer around a sphere which is placed on the bottom wall of a flume. Two way coupling is used to account for the effect of the sphere on the structure of the near-wall turbulence and on the main stream. The calculation of the thermal field is done with the same grid system used for the velocity field. Water and transformer oil were used as test fluids, with the Prandtl numbers Pr = 5.4 and Pr = 55, respectively. The heat transfer calculations were carried out at the constant mean heat flux along the bottom. For both Prandtl numbers the DNS results indicate enhancement of heat transfer coefficient associated with a flow motion toward the wall. The thermal pattern around the sphere is obtained and the effect of Prandtl number is discussed.

During nucleate pool boiling of pure water and water with cationic surfactants, the motion of bubbles and the temperature of the heated surface were recorded by both a high-speed video camera and an infrared radiometer. All experiments were performed at saturated boiling conditions. The boiling curves for various solutions were obtained and compared. The results show that the bubble behavior and the heat transfer mechanism for the surfactant are quite different from those of clear water. In this study the examples of the irregular behavior of nucleation sites and their interactions are presented.

Single crystal diamond (SCD) owns superior mechanical strength, chemical stability, and the highest thermal conductivity among the well-known materials. In this work, we investigated the cooling enhancement of a cold plate made of SCD with embedded microfluidic channels. In particular, we studied the enhanced heat spreading due to conduction followed by convective dissipation of a locally heated resistor mimicking a linear hot spot within electronic chips. Experiments were carried out with various heat fluxes (9-75 W/cm2) and volumetric flow rates (0.02-0.18 ml/min) under transient state. The results showed that cold plate made of a SCD layer with embedded microfluidic channels exhibited the highest cooling effect obtained for maximum applied power density and flow rate. This indicated that combined effect of conductive spreading and convective heat transfer exhibited a significant cooling enhancement. Simulation results further support the improvement of the cooling capability due to the addition of microfluidic channels and the use of SCD as the substrate of the heat sink.

We investigate the impact of the combined diamond heat-spreading layer and microfluidic convection on the performance of a model electronic chip heated locally. Experiments are carried out and a finite element method is used to simulate the thermal response of the device under transient step-wise (without flow) and steady-state (with flow) operation conditions up to heat flux values of 38 and 190 W/cm(2), respectively. In all cases, the temperature on the heated outer silicon surface did not exceed 100 degrees C. The temperature field contour has an oval shape for transient heating without flow and a funnel shape for steady-state heating with flow. For a step-wise heat flux of 38 W/cm(2), the differences between temperatures at the center of the resistor and at the outer surface edge after a time interval of 8 s are 5, 3, and 1 degrees C for the chips without a diamond layer, with a 100-mu m diamond layer, and only a 400-mu m diamond, respectively, which proves the enhanced spreading due to the diamond layer. Under steady-state conditions at a heat flux of 190 W/cm(2) and volumetric flow rates of water between 2 and 5 ml/min, the surface temperature decreases by approximately15% for silicon wafer with a 100-mu mdiamond layer and by approximately 22% for a 400-mu m diamond as compared to heating without the addition of diamond. Of crucial importance is the proximity of the diamond layer to the heat source, which makes this method advantageous over other thermal management procedures, especially for pulsed operating conditions and hot spots.

Underwater explorations along the Israeli coast recovered more than twenty lead braziers, used for cooking aboard, dated to the Roman-Early Byzantine period. Few lead braziers from additional underwater Mediterranean sites were reported. To thermodynamically study the operation of this unique innovation, a steel model of a typical lead brazier was designed and constructed. The model, topped by a copper alloy cooking pot holding water, was used to experimentally analyze the temperatures, combustion, energy balance and efficiency of this “cooking system”. The energy sources were charcoal and fire-wood. Water in the brazier's double-sided walls dispersed and absorbed the heat, preventing thermal melting of the original lead braziers. Maximum temperature measured at the fire bowl's bottom, when burning charcoal and firewood, did not exceed 104°C and 101°C, respectively. Thermal efficiency: the ratio between energy used for heating and evaporation of the water in the cooking pot, to the energy released by the burning fuel, depended on the type and mass of the fuel. It significantly increased from 5.7% to 14.5% with the increase in charcoal mass, from 0.250kg (minimum mass in the present study) to 0.315kg (maximum mass). The thermal efficiency of 0.604kg wood was ~15.8%. Practical cooking using the model was successful. The results provide deeper understanding of the technology used for cooking aboard during the Roman period. Although the available repertoire of materials and technologies was limited relative to modernity, the final result shows the overall competence of Roman craftsman to define an engineering problem and solve it satisfactorily.

•Effect of ultrasound field on subcooled boiling in water and surfactant solutions was studied.•Enhancement of heat transfer from the wire of d = 50 µm and the tube of d = 1.5 mm was investigated.•For d = 50 µm boiling enhancement decreases monotonically with increase in heat flux.•For d = 1.5 mm enhancement of heat transfer almost does not change with increase in heat flux.

Experiments were conducted on subcooled boiling at 30 degrees C on a heated wire of d = 50 mu m in diameter and a heated tube of d = 1.5 mm in diameter considered as micro-scale and macro-scale heaters, respectively. For boiling on the wire of d = 50 mu m submerged in water, jet flow due to surface tension gradient played an important role in promoting heat transfer from the wire to the liquid. For subcooled boiling on the wire of d = 50 mu m submerged in surfactant solutions, jet flow was not observed. The adsorption of surfactants at the vapor-liquid interface of the bubble creates a surface excess concentration gradient, which acts in the opposite direction to the imposed temperature gradient and it results in the reduction of thermo-capillary convection around the bubble.The experimental data obtained under conditions of boiling on the wire of d = 50 mu m in surfactant solutions showed different behavior of boiling curves compared to those obtained for macro-scale boiling. The boiling curves obtained during boiling on micro-scale heater are not dependent on surfactant concentration. When surfactants are present in water, there is generally a reduction of the overall or combined thermo-capillary and diffuse-capillary Marangoni convection. (C) 2015 Elsevier Ltd. All rights reserved.

Cybernetic Transportation System (CTS) is a relatively new branch of the Intelligent Transportation System, which can provide flexible transportation services for university campuses, resorts, and industrial parks. The proposed simulation approach is suitable for a preliminary assessment of CTS-related environmental impact in real-world driving conditions. The model is built to allow performance prediction of a single cybercar or the whole CTS in a wide range of operational conditions. A simple formula is proposed to compare the environmental impact of CTS with that of conventional vehicles. Calculation techniques are developed to evaluate the effect of the relative receptor density and different fuel resources used for electricity generation on the environmental impact of the transportation system based on battery-electric vehicles.

Experimental investigation of a heat sink for cooling of photovoltaic solar cells up to 2000 suns concentration was conducted. Flow boiling of refrigerant HFC-134a in a pin-fin microchannel was investigated in the range of mass flux 220-380 kg/m(2) s, heat flux 30-170 W/cm(2), and an exit vapor quality, x(out),from 0.2 to 0.75. The heat sink was a pin-fin microchannel module installed in an open flow loop. Deviation from the measured area average temperatures was 1.5 degrees C at q = 30 W/cm(2), and 2.0 degrees Cat q = 170 W/cm(2). These results indicate that use of pin-fin microchannel heat sink enables keeping an electronic device near uniform temperature under steady state and transient conditions. The heat transfer coefficient varied significantly with refrigerant quality and showed a peak at an exit vapor quality of 0.55 in all the experiments. At relatively low heat fluxes and vapor qualities, the heat transfer coefficient increased with vapor quality. At high heat fluxes and vapor qualities, the heat transfer coefficient decreased with vapor quality.

Here we investigate experimentally application of single-phase water micro-pin-fin cooler for ‎thermal management of a triple-junction solar cell under concentration of 500 suns. The ‎micro-pin fin heat sink was fabricated of copper using the electric discharge machining‎ (EDM). Silver over nickel ‎plating was applied to the heat sink to allow soldering of the resistor to the heat sink. The heat ‎sink of 1×1 cm 2 has pin-fins of 0.35×0.35 mm, 1 mm height, and pitch of 0.45 mm. The ‎dimensions were taken from typical micro-scale devices described in the literature. To ‎minimize the pressure drop the pin-fins were designed in staggered arrangement, and pinfin ‎corner was normal to the flow direction. Experiments were performed in the range of heat flux ‎q=5.2-24.7 W/cm 2 and mass flux G=10.7-39.1 kg/m 2 s. A thermal high speed imaging ‎radiometer was utilized to study the temperature field on the electrical heater. We compared ‎temperature non uniformity (on the heated surface) under conditions of convective heat transfer ‎of water in micro-channel without pin-fins. With pin-fin cooling, the temperature distribution ‎on the whole heater, as recorded by the IR camera, showed that the standard deviation from ‎the average temperature was 0.3-1.9 K and the maximum difference of the wall temperature ‎between different points did not exceed 3-5 K. By cooling at the same conditions using ‎rectangular micro-channel without pinfins the difference between wall temperature at the ‎inlet and the outlet was about 17-28 K, depending on heat flux and mass flow rate. This study, ‎to the best of our knowledge, is the first presentation of the method that makes use of a uniform ‎temperature microchannel single-phase heat sink‎.

New methods for cooling of microelectronic elements have been recently developed, including application of ultrasonic fields, which can enhance the heat transfer in two-phase cooling. Here we deal with ultrasonic enhancement of heat transfer from wires in subcooled water pool boiling at subcooling of 80 K. The main purpose is to find the effect of wire diameter on the heat transfer coefficient. The wire heaters were used as a source of constant heat flux, and as a thermometers. The experiments were carried out using wires of different diameters: 20, 50, 90, 200 and 0.250 mu m, submerged in a bath of water. The frequency of the ultrasonic field was 40 kHz and the intensity was 0.5 W/cm(2). The wire wall temperature was measured as a function of wire surface heat flux. The effect of acoustic field on heat transfer was measured by the change in the average surface temperature of the heater. When the ultrasonic field was applied, the wall temperature decreased depending on wire diameter and heat flux. Video images of vapor bubble dynamics within the sound field aided in the analysis, and are presented here. The highest heat transfer augmentation was registered for the heated wire of diameter 200 mu m during pool boiling in the ultrasound field. (C) 2014 Elsevier Ltd. All rights reserved.

The thermal characteristics of a laboratory pin-fin microchannel heat sink were empirically obtained for heat flux, q″, in the range of 30–170 W/cm2, mass flux, m, in the range of 230–380 kg/m2 s, and an exit vapor quality, xout, from 0.2 to 0.75. Refrigerant R 134a (HFC-134a) was chosen as the working fluid. The heat sink was a pin-fin microchannel module installed in open flow loop. Deviation from the measured average temperatures was 1.5 °C at q = 30 W/cm2, and 2.0 °C at q = 170 W/cm2. These results indicate that use of pin-fin microchannel heat sink enables keeping an electronic device near uniform temperature under steady state and transient conditions. The heat transfer coefficient varied significantly with refrigerant quality and showed a peak at an exit vapor quality of 0.55 in all the experiments. At relatively low heat fluxes and vapor qualities, the heat transfer coefficient increased with vapor quality. At high heat fluxes and vapor qualities, the heat transfer coefficient decreased with vapor quality. A noteworthy feature of the present data is the larger magnitude of the transient heat transfer coefficients compared to values obtained under steady state conditions. The results of transient boiling were compared with those for steady state conditions. In contrast to the more common techniques, the low cost technique, based on open flow loop was developed to promote cooling using micropin fin sinks. Results of this experimental study may be used for designing the cooling high power laser and rocket-born electronic devices.

Here we present an investigation of boiling incipience in parallel micro-channels and compare the results with those reported for conventional channels. To provide additional insights into the role of surface roughness on the onset of nucleate boiling (ONB) in micro-channels, the roughness parameters were studied extensively. Onset of nucleate boiling is investigated in uniformly heated parallel rectangular micro-channels of Dh=297μm with subcooled water flow at mass flux of 15.4–77.1kg/m2s. It is shown that in the literature significant disagreement between values of wall temperature and the average mass liquid temperature at ONB point is due to different experimental conditions. For the analysis of the conditions at which the ONB occurred, the parameter based on the relation of the difference between wall excess temperature and bulk fluid temperature at ONB to the difference between saturation temperature and fluid inlet temperature is developed. The experimental results indicate that parameters, which affect incipience of nucleation in micro-channels, such as cavity radius and wall excess temperature, are well predicted by the theoretical nucleation criteria, which were developed for conventional size channels.