Minimum Wall Temperature Research Articles

Thermodynamic effects of gas adiabatic index on cavitation bubble collapse

In this paper, an improved multicomponent lattice Boltzmann model is employed to investigate the impact of the gas properties, specifically the gas adiabatic index, on the thermodynamic effects of cavitation bubble collapse. The study focuses on analyzing the temperature evolution in the flow field and the resulting thermal effects on the surrounding wall. The accuracy of the developed model is verified through comparisons with analytical solutions of the Rayleigh-Plesset equation and the validation of the adiabatic law. Then, a thermodynamic model of cavitation bubble composed of two-mixed gases collapsing near a wall is established to explore the influence of the gas adiabatic index γ on the temperature behavior. Key findings include the observation that the γ affects the temperature of the first collapse significantly, while its influence on the second collapse is minimal. Additionally, the presence of low-temperature regions near the bubble surface during collapse impacts both bubble and wall temperatures. The study also demonstrates that the γ affects maximum and minimum wall temperatures. The results have implications for selecting specific non-condensable gas properties within cavitation bubbles for targeted cooling or heating purposes, including potential applications in electronic component cooling and environmental refrigeration.

Oriented square shaped pin-fin heat sink: Performance evaluation employing mixture based on ethylene glycol/water graphene oxide nanofluid

Based on effective heat dissipation in electronic devices, the current study deals with heat transfer characteristics of oriented square shaped pin fin heat sink having a fin angle of 22.5°. It has been investigated experimentally employing graphene oxide (GO) nanofluid under laminar flow regime. The performance of graphene oxide (GO) nanofluid having concentrations of 0.004%, 0.0065%, and 0.009 vol% is compared with a mixture of EG-water (50:50) as base fluid under three different heating powers of 20 W, 35 W, and 50 W. The experimental results revealed better thermal performance for all used concentrations of (GO) nanofluid compared to the base fluid due to the heat transfer augmentation and reduction in thermal resistance as a result of enhanced thermal conductivity of nanofluid. It was observed that the thermal performance of heat sink improved with increasing Reynolds number and heating power. Minimum wall temperature and maximum enhancement in Nusselt number of 30.75 °C and 19.68% respectively is associated with graphene oxide nanofluid having a concentration of 0.009 vol%. According to the results, nanofluid exhibited better trends of thermal enhancement in comparison of EG/water base fluid with an increase in nanoparticle concentration usually at low Reynolds number. Thus, graphene oxide nanofluid has proven to be the potential candidate for improving the thermal performance of heat sink.

Effect of Cross-Sectional Geometry on Hydrothermal Behavior of Microchannel Heat Sink

Abstract The aim of this paper is to numerically analyze the hydrothermal behavior of different cross-sectional geometries of microchannel heat sinks (MCHSs) and conduct a comparative analysis of traditional and non-traditional designs using ANSYS Fluent. It is expected that the proposed design discussed in this paper will improve the performance of MCHSs by maximizing the cooling capability and minimizing the thermal resistance and entropy generation rate, thus leading to better energy efficiency. The channel designs include a rectangular microchannel (RMC), a circular microchannel (CMC), an elliptical microchannel (EMC), a trapezoidal microchannel (TMC), a hexagonal microchannel (HMC), and a new microchannel (NMC) which has a plus-like shape. The discussed geometry of the NMC is designed in such a way that it maximizes the cross-sectional area and the wetted perimeter of the channel, keeping the hydraulic diameter constant ( D h = 412 {D_{h}}=412 µm). The performance of various channels is compared on the basis of pressure drop, wall temperature, thermal enhancement factor, thermal resistance, thermal transport efficiency, and entropy generation rates. It has been observed that the NMC is capable of cooling effectively and it can achieve a minimum wall temperature of 305 K, thus offering the lowest thermal resistance ( R th {R_{\mathrm{th}}} ), irreversible heat loss, and entropy generation rate. Moreover, the NMC has achieved the highest value of the thermal enhancement factor, i. e., 1.13, at Re = 1 , 000 \mathrm{Re}=1,000 . Similarly, it has the highest thermal transport efficiency of almost 97 % at Re = 1 , 000 \mathrm{Re}=1,000 , followed by the TMC and the RMC. Overall, the NMC has achieved the best performance in all aspects, followed by the RMC and TMC. The performance of the EMC, the CMC, and the HMC was found to be the worst in this study.

Application of a novel hybrid nanofluid containing graphene–platinum nanoparticles in a chaotic twisted geometry for utilization in miniature devices: Thermal and energy efficiency considerations

This study aims to research the flow, heat transfer and energy efficiency of a novel hybrid nanofluid containing graphene nanoplatelets decorated with platinum nanoparticles in a chaotic twisted minichannel. This geometry develops chaotic advection owing to geometrical perturbations. Dean roll-cells are formed in the channel, and are the cause for irregular trajectories of fluid particles. The channel includes four bends, such that the chaotic motions are strengthened gradually along the bends, and reach a maximum rate at the exit of them, which causes minimum wall temperature in those regions. To better evaluate the merit of chaotic channel, simulations are also performed in a simple channel. The convective heat transfer and pumping power in both channels augment by increasing either concentration or Dean number. The chaotic channel has higher heat transfer and pressure drop than the simple one. Due to the excellent attributes of graphene, not only heat transfer enhances significantly but also pressure drop does not increase noticeably by adding nanoparticles. Figure of merit, i.e. ratio of enhancement in convective hear transfer to enhancement in pumping power in the case of employing the chaotic channel rather than the simple channel, is always greater than 1.5, which demonstrates a promising outcome.

Analysis of nozzle designs on zoned and staged double P-type gas-fired radiant tube

In this study, the nozzle characteristics of a zoned and staged double P-type gas-fired radiant tube were improved to optimize combustion efficiency. Experimental and numerical analyses were conducted on the optimized radiant tube. The deviation between the experimental and simulated monitoring parameters was less than 3.6%, thereby proving the reliability of the model. The gas and wall temperatures were compared by changing the positions of the main and branch tube nozzles, and structural parameters. The results showed that as the nozzle moved outward, the maximum surface temperature decreased from 1328K to 1320K, and the minimum wall temperature increased from 1253K to 1258K. When the branch tube nozzle was located at the intersection of the tee and branch tubes, the impact of high-temperature gas on the tube wall was reduced, and the uniformity of the branch radial temperature was improved. When the nozzle was fully premixed, the temperature difference was at its minimum. When the tube nozzle was asymmetric, the wall temperature difference was at its minimum, and combustion thermal efficiency was at its maximum.

Experimental investigation of heat transfer from a 2 × 2 rod bundle to supercritical pressure water

Heat transfer experiments with supercritical pressure water flowing vertically upward through a 2×2 rod bundle have been performed at Xi’an Jiaotong University. A fuel-assembly simulator with four heated rods was installed inside a square channel with rounded corner. The outer diameter of each heated rod is 8mm with an effective heated length of 600mm. The experiments covered the pressure range of 23–28MPa, mass-flux range of 350–1000kg/(m2s) and heat-flux range on the rod surface of 200–1000kW/m2. Heat transfer characteristics of supercritical pressure water through the bundle were examined with respect to variations of heat flux, system pressure, and mass flux. These characteristics were shown to be similar to those previously observed in tubes or annuli. The experimental data indicate a non-uniform circumferential wall-temperature distribution around the heated rod. A maximum wall temperature was observed at the surface facing the corner gap between the heated rod and the ceramic tube, while the minimum wall temperature was observed at the surface facing the center subchannel. The difference between maximum and minimum wall temperatures varies with heat flux and/or mass flux. Eight heat transfer correlations developed for supercritical water were assessed against the current set of test data. Prediction of the Jackson correlation agrees closely with the experimental Nusselt number. A new correlation has been derived based on Jackson correlation to improve the prediction accuracy of supercritical heat transfer coefficient in a 2×2 rod bundle.

Heat Transfer in a Porous Electrode of Fuel Cells

The thermal-fluid behaviors in a porous electrode of a proton exchange membrane fuel cell (PEMFC) in contact with an interdigitated gas distributor are investigated numerically. The porous electrode consists of a catalyst layer and a diffusion layer. The heat transfer in the catalyst layer is coupled with species transports via a macroscopic electrochemical model. In the diffusion layer, the energy equations based on the local thermal nonequilibrium (LTNE) are derived to resolve the temperature difference between the solid phase and the fluid phase. Parametric studies include the Reynolds number and the Stanton number (St). Results show that the wall temperature decreases with increasing Stanton number. The maximum wall temperatures occur at the downstream end of the module, while the locations of local minimum wall temperature depend on the Stanton numbers. Moreover, the solid phase and the fluid phase in the diffusion layer are thermally insulated as St⪡1. The diffusion layer becomes local thermal nonequilibrium as the Stanton number around unity. The porous electrode is local thermal equilibrium for St⪢1. Finally, the species concentrations inside the catalyst and diffusion layers are also provided.

Conjugate heat transfer and particle deposition in the modified chemical vapor deposition process: Effects of torch speed and solid layer

A study has been carried out for the conjugate heat transfer and particle deposition that occur during the modified chemical vapor deposition (MCVD) process. The analysis includes thermophoretic particle transport in the gas flow and heat transfer through the solid layer; the effects of variable properties in both the gas and the solid regions are included. A notable feature of the study is the inclusion of the effects of periodic heating due to the repeated traversing of the torch and the effects of the increasing solid layer thickness as the particles deposit. A new concept uses a two torch formulation to simulate the torch heating from both the present and the previous passes. This formulation is able to predict the minimum wall temperature in front of the torch, which is closely related to the deposition efficiency. Localized heating of the moving torch is studied using the heat flux boundary condition on the tube wall. The calculated surface temperature distribution and the deposition efficiency are in good agreement with experimental data. Of particular interest are the effects of torch speed and solid layer thickness on the efficiency, the rate of deposition of the particles and the taper length.

STABILITY ANALYSIS FOR THE PREDICTION OF THE MINIMUM TEMPERATURE OF FILM BOILING

Abstract The stability of film boiling on a vertical, constant temperature wall is analyzed by a steady-state small perturbation method. It was found that large amplifications of the perturbations in the downstream direction are predicted when the vapor Reynolds number falls below a critical value, indicating a great tendency to collapse the vapor film. Using a relatively simple model for the film thickness, the minimum wall temperature required to sustain the vapor film is calculated. The results show a tendency which is in agreement with experimental observation. It is suggested, however, that using a more accurate model for the film thickness can lead to a good prediction of the minimum wall temperature and explain the large effect of liquid subcooling, system pressure, surface tension and other fluid properties.

Non-Newtonian fluid laminar flow and forced convection heat transfer in rectangular ducts

Numerical solutions for fully developed laminar flow forced convection heat transfer of a power law non-Newtonian fluid in rectangular ducts are presented. Finite difference methods are developed for the governing equations to obtain the velocity and temperature distributions. Friction factor results are given for flow through rectangular ducts of aspect ratios of 0.2, 0.5 and 1.0 with power law index n values of 0.5 to 1.0. For the same flow conditions the Nusselt values, maximum wall temperatures, and minimum wall temperatures for the H2 thermal boundary condition for different combinations of heated and adiabatic walls are obtained. Also the Nusselt values for slug flow ( n=0 ) are presented for the H2 boundary condition.

Further Contributions to the Study of the Leidenfrost Phenomenon

Experimental values of both the minimum wall temperatures for spheroidal state and the total vaporization times of water, n-octane and carbon tetrachloride drops are presented. A novel apparatus was used and tests were run in a wall temperature range scarcely investigated by other authors. Vaporization times are compared with correlations accounting for the diffusive evaporation and these are suggested to be applied in ranges different from those recommended by the authors.

Heat Conservation System for Arc Lamps

In order to have high efficiency in a metal arc discharge, the metal vapor pressure has to reach an optimum value, and this requires a minimum wall temperature. Therefore, if one can effectively use the thermal energy for heating purposes, the efficiency of the lamp can be improved. In this paper the author will discuss the effectiveness of selecting a low emissivity material for the arc tube, and reducing the effective emissivity of arc tubes by using transparent heat shields to conserve heat. Simplified formulas for the wall temperature calculations with such heat conservation systems are included.

Design and Testing of a Heat Transfer Model of a Raccon (Procyon Lotor) in a Closed Tree Den

A heat transfer model of a raccoon (Procyon lotor) in a hollow tree was designed and tested by simulating an overwintering raccoon with a fur—covered, electrically—heated cylinder in a closed den. A comparison was made between predicted and observed fur surface temperatures, and heat losses from the top, side, and bottom. Under these conditions, transfer coefficients derived from engineering relationships can be dependably used in the field; the average difference between predicted and observed fur surface temperatures was 1.6°C, and the sum of the predicted fluxes through the surfaces of the model was within 5% of the measured total. Total heat loss increased approximately linearly as inside wall temperature decreased, but the contribution by convection and radiation to the total changed substantially. As inside wall temperature decreased, convective flux was an increasing percentage of the total, whereas radiation loss decreased to a minimum value of 56% of the total flux at the minimum wall temperature. Twenty—five percent of the total flux was lost from the top, 62% from the side, and 13% from the bottom. More than 65% of the total resistance to heat loss through the model side was in the fur. The air space presented 17%, with the rest in the den wall With respect to heat loss through the base of the den, only 6% of the total resistance was in the compressed fur. But because of the high resistance of the den base, the loss was half that of the top. Small variations in den wall thickness are not a major factor in reducing heat loss since the wall accounts for only about 18% of total heat flow resistance. Energy loss and average monthly weather screen air temperature are correlated.