Single-phase Heat Transfer Coefficient Research Articles

Effect of vertical vibration on transient heat transfer characteristics of flow boiling in small rectangular channels

In the thermal management of fuel cell vehicles, radiators are subjected to vibrations due to vehicle speed and road surface irregularities. However, there has been no comprehensive study of the impact of these vibrations on the heat transfer performance of radiators. Therefore, this paper employs R141b to conduct three-dimensional transient numerical simulations of flow boiling in small channels under vertical vibrations. The study reveals that the single-phase heat transfer coefficient exhibits periodic variations. At the same vibration frequency, the heat transfer frequency for the vertically oriented heating surface is twice that of the horizontally oriented surface, This is because the thermal boundary layer varies with the vibration velocity. Furthermore, as the frequency increases to 21 Hz, the time-averaged heat transfer coefficient can increase by up to 2.5 times. Following boiling onset, for horizontally oriented heating surfaces, an increase in frequency or amplitude enhances the transient heat transfer performance. Under the conditions of 28 Hz and 5 mm, the heat transfer coefficient can increase by 125.7%. However, for vertically oriented heating surfaces, when the frequency exceeds 14 Hz or the amplitude exceeds 8 mm, the heat transfer performance weakens after 7.143 s due to the promotion of bubble coalescence by the vibrations.

Boiling heat flux partitioning model with bubble tracking method considering bubble merger and stochastic characteristics

This study presents the development and validation of a numerical tool of heat partitioning model for boiling heat transfer using a bubble tracking method. It was aimed to account for the effect of stochastic phenomena in the boiling process by simulating individual bubbles. Saturated pool boiling conditions on a horizontal plate heater were analyzed to demonstrate the effectiveness of the proposed model. The model was devised based on the observations from high-resolution pool boiling experiments, assuming a truncated sphere for bubble shape and using simplified microlayer size and thickness models. The developed heat partitioning model was verified by comparisons with the conventional heat partitioning model. Afterward, the validation of the model was conducted by comparing the result of the simulation with the experimental data of the saturated pool boiling. In this validation, measured values of nucleation site density, departure diameter, single-phase heat transfer coefficients, etc. were applied. Additionally, the effect of stochastic phenomena on boiling heat transfer was evaluated using the bubble tracking method, focusing on nucleation timing and nucleation site distribution. Stochastic nucleation was found to reduce the bubble merger probability compared to simultaneous nucleation, leading to increased microlayer evaporation. Stochastic site distribution, on the other hand, decreased active nucleation sites due to the site suppression effect, resulting in reduced microlayer evaporation compared to uniform site distribution.

High-performance thermal management system for high-power LEDs based on double-nozzle spray cooling

Thermal management is one of the main challenges for high-power LEDs. As one of the state-of-the-art thermal management approaches, spray cooling can meet the requirements of fast cooling speed, large heat transfer coefficient, and less energy consumption when it provides thermal management for electronics with high heat flux. In this study, a high-performance spray cooling system for high-power LEDs is designed and established. The influence of nozzle configuration, flow rate, and nozzle-to-surface distance on the heat transfer characteristics are experimentally and numerically investigated. The system shows excellent heat dissipation capacity. Two different nozzle configurations are compared and the results show that the double-nozzle configuration possesses the highest single-phase heat transfer coefficient, reaching up to 20.7 kW/m2·K with the flow rate of 150 mL/min. Interestingly, double-nozzle, which owns better heat transfer performance compared to single-nozzle, shows worse temperature uniformity inside the testing module. Simultaneously, the junction temperature of double-nozzle at the center chip can also be controlled to 85 °C corresponding to a 10% reduction for single-nozzle. The proposed cooling system may enable the further application of spray cooling in electronics like high-power LEDs, potentially reducing the energy consumption in cooling electronics and promoting the utilization of low-grade thermal energy.

A Comprehensive Compendium on Passive Augmentation Techniques for Enhancement of Single-Phase Heat Transfer Coefficients in Heat Exchanger Tubes under Laminar and Turbulent Flow Conditions

Several passive techniques can substantially improve the heat transfer performance of conventional heat exchangers. Passive methods persistently use heat transfer augmentation inside tubular sections, which tackle dominant thermal resistance. The thermal resistance in turbulent flow is primarily due to a thin viscous sublayer near the tube wall. This proliferates into the cross-section of the conduit in the case of laminar flows due to the existence of a relatively thicker boundary layer. This urges disturbance in the entire fluid throughout the cross-section in laminar flows, whereas augmentation devices are usually located close to the wall for turbulent flows. Most of these methods yield increased fluid residence time within the system by inducing swirling motion. The form of disturbance in the flow field is the characteristic of the passive technique used to prolong the fluid residence time and is exhibited differently in both flow regimes. The present article showcases a comprehensive review of heat transfer enhancement through thermo hydraulic performance assessment of these methods reported in the literature. The comparison is based on ratios of Nusselt numbers, first at the same Reynolds numbers, and then, at equal pumping power with constant heat transfer area.

INVESTIGATION OF SINGLE-PHASE FLOW CHARACTERISTICS IN A STAGGER PIN-FINS COMPLEX GEOMETRY

The cooling equipment project must use electrical and electronic equipment because of the need to remove the heat generated by this equipment. Investigation; R-113 single-phase flow heat transfer; (50 x 50 mm2) cross-section and (5 mm) height; used in a series of stagger-square micro-pin fins. Inlet temperature of (25 °C); (6) Mass flow rate at this temperature, the recommended range is (0. 0025 -0.01 kg/sec) the inlet and outlet pressures are approximately (1-1.10 bar), and through (25- 225 watts) applied heat. The iterative process is used to obtain the heat flow characteristics, for example; the single-phase heat transfer coefficient is completely laminar flow developing, in this flow, guesses the wall temperature, guess the fluid temperature. The possible mechanism of heat transfer has been discussed.

Correlations for heat transfer characteristics of the onset of nucleate boiling in submerged jet impingement

The submerged jet impingement boiling is one of the most effective heat transfer methods in dissipating both transient and steady concentrated heat loads. However, studies on the submerged jet impingement boiling still lack experimental data, especially for the heat transfer characteristics of the onset of nucleate boiling (ONB). Therefore, an experimental study of a submerged circular jet impinging on a polished copper surface is conducted in the subcooled water. The experiments investigate the steady-state heat transfer characteristics at the stagnation point from single phase convection to partial nucleate boiling. The effects of jet Reynolds number, liquid subcooling, and nozzle diameter on the onset of nucleate boiling are systematically studied. The experimental results show that the single phase convective heat transfer can be enhanced by increasing jet velocity at a fixed nozzle diameter or decreasing nozzle diameter under the same jet Reynolds number. The liquid subcooling seems to have negligible effect on the single phase heat transfer coefficient. For the ONB point, it is found that the ONB wall superheat is not influenced by liquid subcooling or jet parameters. The ONB heat flux, on the other hand, is found to depend on the experimental parameters. Increasing the jet Reynolds number and liquid subcooling, or decreasing the nozzle diameter could broaden the single phase convective regime and delay the onset of nucleate boiling, in terms of heat flux. Empirical correlations are proposed for predicting the stagnation point heat transfer coefficient, heat flux, and wall superheat at the ONB point. The comparison between the predicted values and the experimental data indicates that the empirical correlations are able to predict the heat transfer characteristics of the onset of nucleate boiling within an acceptable accuracy under the applicable range.

Investigation of Single-Phase Flow Characteristics in an Inline Pin-Fins Complex Geometry

In this guess investigation of concerns R113 heat transfer of pressure drop flow in single-phase flow in inline on the square shape of micro-channel of pin-fins own a (5×5 mm2) cross-area segment section via of (5 mm) heightly. Therefore, the inline square micro-channel of pin-fins complex geometry additionally owning (25) numbers of the pin-fins, total pin-fins channel dimension (50 mm × 50 mm). Subsequently. The liquid has (25°C) inlet temperature for the same inlet temperature six mass flow rates were applied, ranging from (0.0025 - 0.01 kg/sec), and heat apply ranged from (40-200 Watts). Guess single-phase heat transfer coefficients, guess liquid temperatures and guess wall temperatures were reported in this investigation.

Simulation of flow boiling in micro-channels: Effects of inlet flow rate and hot-spots

This paper presents the findings of a numerical study on the flow boiling in a micro-channel heat sink. The Navier-Stokes equations, energy equation, and the continuity equation are solved in a finite-volume framework using the front-tracking method. The numerical method is validated by comparison with the experimental results for a slug bubble growth, and vertical flow boiling. The numerical method is then used to study the effect of changing the inflow mass-velocity on the heat transfer coefficient, bubble size distribution, and the bubble nucleation frequency for a constant heat flux. The mean heat transfer coefficient of all the cases is found to be nearly twice that of the single-phase heat transfer coefficient. The bubble nucleation frequency is found to increase monotonically with the inflow mass-velocity. The bubble size distribution along the channel is found to become flatter as the mass-velocity is increased. We identify three distinct phases of the bubble evolution, namely the initial rapid growth phase, the boiling dominant phase, and finally the condensation dominant phase. Subsequently, the numerical method is used to study the effect of having a hot-spot near the bubble nucleation site on the heat transfer characteristics. It is found that the bubble nucleation frequency increases and the bubbles’ maximum volume decreases as the intensity of the hot-spot is increased for a fixed inlet flow rate. It is also observed that the average heat transfer coefficient does not change significantly with changing the intensity of the hot-spot, and that the bubble size distribution along the channel becomes flatter as the intensity of the hot-spot is increased.

Two-Phase Flow Heat Transfer in Micro-Fin Tubes

An experimental investigation of single-phase and two-phase (evaporation and condensation) heat transfer and pressure drop characteristics is performed for five micro-fin tubes with various geometries using refrigerant R22 as the working fluid. The mass flux is in the range of 130–550 kg/m2s. Evaporation tests are conducted at 6 °C saturation temperature, 0.1 inlet quality and 0.9 outlet quality, while condensation tests are carried out at 45 °C saturation temperature, 0.8 inlet quality and 0.1 outlet quality. The results suggest that the micro-fin tubes single-phase heat transfer coefficients increased by 46.3%–63.6% compared to smooth tubes. Tube geometries that have the best evaporation or condensation heat transfer performance are identified. A new evaporation heat transfer correlation that takes the micro-fin effects and heat transfer mechanisms into consideration is developed, which predicts the experimental data within 15% error compared with the previously reported correlations which show prediction errors beyond 30%. The new model is applied to intermittent and annular flow patterns in micro-fin tubes. As for condensation, the Choi et al. correlation well predicts the pressure drop within 20% error, and the Yu and Koyama correlation provides the best heat transfer predictions within 25% error.

An experimental investigation on the heat transfer and friction coefficients of a small plate heat exchanger with chevron angle

In the present study, a small brazed and commercial plate heat exchanger (PHE) with chevron angle 60° and symmetrical layout, was experimentally investigated. New correlations for calculating heat transfer and friction coefficients of hot flow channels were presented. Few studies have been done on the small PHEs and due to the industrial needs, it is necessary to conduct new researches in this field. For this purpose, an experimental model was designed to calculate the single-phase water flow, heat transfer, and friction coefficients. The previously presented correlations were not successful in predicting the mentioned coefficients for the small PHEs and showed a 34% difference with the results of this study. Chevron grooves on the PHE plate can enhance the heat transfer as well as increasing the pressure drop. Defining $$ \left(\frac{j}{f}\right) $$ factor, it was revealed that with the increase of Reynolds number, heat transfer increased more than friction. In previous studies, with increasing Reynolds, $$ \left(\frac{j}{f}\right) $$ factor is almost constant in large PHEs but the results of the present study showed that this increase may be 5-fold higher in the small PHEs as compared with the larger ones. As there was no temperature and heat transfer coefficient value in the hot flow channels of PHE, the Nusselt number was calculated by the modified Wilson plot method.

Operational behavior and heat transfer in a thermosiphon desorber at sub-atmospheric pressure. Part I: The model

The thermosiphon desorber or bubble pump is advantageous in absorption cooling devices, since the processes of refrigerant desorption and solution pumping is combined in one hermetical unit. This supersedes the necessity of a mechanical solution pump. In the application with NH3/H2O the thermosiphon desorber typically operates at a pressure of around 15 bar with a liquid vapor density ratio in the range of 102, which is in a well-known regime of two phase flow. Nevertheless, for the application of thermosiphon desorbers in H2O/LiBr absorption chillers, which are commonly operated at a pressure below 0.1 bar with the liquid vapor density ratio in the range of 104, there is a lack of heat transfer correlations for two-phase flow.First attempts to fill this gap of flow boiling heat transfer correlations in the thermosiphon desorber at sub-atmospheric pressures is presented in this paper. A model is formed by solving simultaneously the mass, energy, and momentum equations. The well-known correlations for predicting single-phase and two-phase flow heat transfer coefficient and frictional pressure drop have been reviewed to find a possibility for applying them to the thermosiphon desorber. The mass flow rate and heat transfer behavior with the different flow length of the riser tube and with changing the pressure has been theoretically studied in the model. For verification, the model has been compared to experimental data using pure water. This will be presented in the second part of the paper.

Two Phase Flow Through Micro Channel Using Nanofluids

Micro/mini channel based circuits with cooling media is one of the most efficient solution for the thermal management of electronic components due to its higher heat transfer coefficient and higher heat transfer surface area per unit volume. Fluid flow and heat transfer characteristics of flow through microchannel under single phase and two phase flow conditions using Aluminium oxide nanofluid is experimentally investigated. An experimental set up with microchannel heat sink has been fabricated and the analysis has been performed for different volume percentage of nanoparticles like 0.001%, 0.01% and 0.1%.The single phase and two phase heat transfer coefficients were found to be high for flow with nanofluid compared to base fluid but with a pressure drop penalty. The single phase heat transfer coefficient increases with increasing concentration of nanofluid. But the two phase heat transfer coefficient increases up to certain value of the volume concentration of nanofluid and then decreases. The critical heat flux also found to be high for the flow with nanofluid than water and is found to be increasing with increasing concentration of nanofluid. The single phase flow results using water as the working fluid are compared with the standard correlations available in the literature and are found to be matching well.

Numerical simulation and experiment for R124-DMAC bubble absorption process in a vertical tubular absorber

A comparing investigation between the simulated model and experimental verifications for the R124 (2-chloro-l,l,l,2,-tetrafluoroethane)-DMAC (N′, N′- dimethylacetamide) bubble absorption process in a vertical tubular absorber were performed. The objective of the investigation was to model the R124-DMAC bubble absorption process and to verify the model results with visual and non-visual experiments under similar operating conditions. The comparing results between the model and experiments were consistent well. The two-phase and single-phase convective heat transfer coefficient and two-phase mass transfer coefficient enhanced with increase of vapor and solution flow rates, and decreased with increase of solution inlet temperature, solution inlet mass fraction and nozzle orifice diameter. The two-phase mass transfer coefficient varied slightly with vapor flow rates rising. Finally, new correlations of the bubble absorption height, two-phase and single-phase convective heat transfer coefficients and two-phase mass transfer coefficient for R124-DMAC bubble absorption process in a vertical tubular bubble absorber were proposed. All of the correlation equations could predict about 90% of these parameters at a margin of error less than 15% under the operating conditions.

Modelling and simulation of a pillow plate thermosiphon reboiler

Thermosiphon reboilers are the most widely used evaporators in the process industry. Experimental studies suggested a superior performance of pillow plate thermosiphon reboilers over conventionally used tubular designs. This contribution presents a comprehensive model to simulate fluid dynamic as well as thermal performance of pillow plate thermosiphon reboilers. Experimental heat transfer coefficients are extracted from measurements at a thermosiphon reboiler test rig. With this information, correlations for the single phase heat transfer coefficient and the heat transfer coefficient with nucleate boiling as well as convective boiling are presented. A correlation for the heat transfer coefficient for film condensation inside the pillow plate and the two-phase pressure drop between the pillow plates is presented. These correlations are implemented in a modular structured simulation program. Good agreements were shown for the heat transfer with water experiments. The fluid dynamic could be described satisfactory with larger uncertainties at small circulation rates.

Single-phase heat transfer of multi-droplet impact on liquid film

Multi-droplet impact on a thin liquid film covering a heated wall is numerically studied using a three-dimensional model with an implement of a random disturbance subjected to Gaussian distribution. Results show that successive impact results in higher splashing threshold for the trailing droplet than the leading one due to radial flow inside the residual film. Single-phase heat transfer coefficient in the impinged region is remarkably higher than the undisturbed film region, caused by enhanced convective heat transfer. A heat transfer blind spot situated under the central liquid sheet in simultaneous impact is identified, at which relatively high local temperature is observed and heat is more difficult to be rejected due to flow stagnation there. Cooling of the heated wall in successive impact is mainly achieved by contacting of the trailing droplet with the heated wall, rather than mixing of cold droplets and warm film.

Single Phase Heat Transfer In Parallel Micro-Channel Heat Sink

A copper parallel channels test piece has built into the test section containing twenty five, one mm by one mm, parallel channels the channels were fifty mm long; the Heat-transfer coefficients for single-phase are reported. Micro- channel was Square shape and the test section has a glass top plate to permit visual observations. The data are taken while the rig is working .The test section is received the heat by an electric heater is associated normally with a boundary condition for constant heat flux. Because the important variation in the single-phase - heat-transfer coefficient in the inlet zone, the interceding copper and aluminum material is shown to make the test section close isothermal wall boundary condition. The heat conduction influence is taken into computation in the data analysis finally; the effective heat flux and the single-phase heat transfer coefficient are reported in this paper experiments and predictions. Key words— Single phase Heat transfer cofficients , Parallel channels ,Heat flux,Mass flux.

An experimental investigation on flow boiling heat transfer enhancement using Cu-TiO2 nanocomposite coating on copper substrate

Flow boiling heat transfer performances of Cu-TiO2 nanocomposite coated copper surfaces have been studied experimentally in this work for its potential use in heat transfer applications. Experimental studies are performed in a bottom surface heated minichannel with DI water as the coolant. Thin Cu-TiO2 nanocomposite coated copper surfaces are developed using electrocodeposition technique. Coated surfaces developed through this technique have varied surface properties such as wettability, porosity, crystallinity, etc. The developed coated surfaces are characterized with respect to porosity, mean pore diameter, roughness, contact angle, and coating thickness. Flow boiling heat transfer experiments are performed at different mass fluxes in an experimental setup developed for this purpose. The results obtained with bare copper surface are used as the reference data for comparing the performances of the coated surfaces. The Cu-TiO2 coated copper surfaces are found to augment single-phase heat transfer coefficient slightly, whereas the enhancement in the two-phase region is up to 94% depending on the mass flow rate of coolant and heater surface temperature. The critical heat flux (CHF) also augments for the nanocomposite coated surfaces up to 92%. The augmentation in CHF and heat transfer coefficient of Cu-TiO2 coated surfaces is due to the enhanced surface roughness, wettability improvement, and presence of high-density active nucleate site on the surface. In order to have appreciation of the results obtained in present work, the boiling curves and CHF against mass flux for the bare and coated surfaces are compared with previously published experimental results. Thus, the arrangement of the minichannel and porous surface offers a capable choice for compact small size cooling devices due to decreased wall superheat temperature, high CHF, and high heat transfer coefficient.

Simple and general correlation for heat transfer during flow condensation inside plain pipes

This work proposes a new general and simple model to determine the local flow condensation heat transfer coefficient inside plain pipes. The model considers two regimes corresponding to high mass fluxes and/or high thermodynamic qualities and low mass fluxes and/or low thermodynamic qualities. For each region, a new model is suggested which resembles the single-phase heat transfer coefficient model but defining an equivalent Reynolds number in terms of the sum of the superficial liquid and vapour Reynolds numbers. The models consider that the superficial vapour Reynolds number plays a major role in controlling the heat transfer coefficient. The model is able to predict the heat transfer coefficient from channels with a hydraulic diameter of 67 μm up to pipes with a hydraulic diameter of 20 mm for several fluids. No noticeable effect of the diameter of the channel, shape or fluid properties on the heat transfer coefficient has been observed for the studied cases.

Heat transfer coefficient of near boiling single phase flow with propane in horizontal circular micro channel

Experimental study of heat transfer coefficient of single phase has conducted successfully. Technology to reduce energy consumption is important to be investigated, moreover with implementation of renewable energy. The study used natural refrigerant of propane as working fluid with heating process. The aim of the present research is to study near boiling single phase heat transfer coefficient on the micro channel of 500 μm diameter and 0.5 meter length. Variable of research are heat flux of 1 to 15 kW/m2, mass flux of 297 to 1102 kg/m2.s, and test section inlet temperature of 21 to 26°C. The experimental results showed single phase laminar-turbulence flow with Reynolds number less than 10000. Heat transfer coefficient of near boiling single phase flow is significantly affected by Reynolds number and Prantl number. Using the present experimental data, new correlation of Nusselt number is developed as a function of Reynolds number and Prantl number.

Condensation of binary mixtures on horizontal tubes

The two most common models to describe the condensation of binary mixtures are the equilibrium model by Silver (Trans Inst Chem Eng 25:30–42, 1947) and the film model by Colburn and Drew (Transactions of the American Institute of Chemical Engineers 33:197–215, 1937), which is stated by Webb et al. (Int J Heat Mass Transf 39:3147–3156, 1996) as more accurate. The film model describes the outer heat transfer coefficient by subdividing it into two separate resistances against the heat transfer. The resistance of the liquid condensate film on the tube can be calculated with equations for the condensation of pure substances for the analogous flow pattern and geometry using the property data of the mixture. The resistance in the gas phase can be described by a thermodynamic parameter Z and the single phase heat transfer coefficient αG. In this work measurements for condensation of the binary mixtures n-pentane/iso-octane and iso-propanol/water on horizontal tubes for free convection are carried out. The obtained results are compared with the film model by Colburn and Drew (Transactions of the American Institute of Chemical Engineers 33:197–215, 1937). The comparison shows a rather big deviation between the theoretical model and the experimental results. To improve the prediction quality an own model based on dimensionless numbers is proposed, which describes the experimental results of this work significantly better than the film model.