Applying Heat Transfer Coefficient Data to Electronics Cooling

This paper presents selected results on heat transfer to supercritical water flowing upward in a 4-m-long vertical bare tube. Supercritical water heat-transfer data were obtained at pressures of about 24 MPa, mass fluxes of 200 – 1500 kg/m2s, heat fluxes up to 884 kW/m2 and inlet temperatures from 320 to 350°C for several combinations of wall and bulk-fluid temperatures that were below, at or above the pseudocritical temperature. In general, the experiments confirmed that there are three heat-transfer regimes for forced convective heat transfer to water flowing inside tubes at supercritical pressures: (1) normal heat-transfer regime characterized in general with heat transfer coefficients (HTCs) similar to those of subcritical convective heat transfer far from critical or pseudocritical regions, which are calculated according to the Dittus-Boelter type correlations; (2) deteriorated heat-transfer regime with lower values of the HTC and hence higher values of wall temperature within some part of a test section compared to those of the normal heat-transfer regime; and (3) improved heat-transfer regime with higher values of the HTC and hence lower values of wall temperature within some part of a test section compared to those of normal heat-transfer regime. These new heat-transfer data are applicable as a reference dataset for future comparison with supercritical-water bundle data and for a verification of scaling parameters between water and modeling fluids. Also, these HTC data were compared to those calculated with the original Dittus-Boelter and Bishop et al. correlations. The comparison showed that the Bishop et al. correlation, which uses the cross-section average Prandtl number, represents HTC profiles more correctly along the heated length of the tube than the Dittus-Boelter correlation. In general, the Bishop et al. correlation shows a good agreement with the experimental HTCs outside the pseudocritical region, however, overpredicts the experimental HTCs within the pseudocritical region. The Dittus-Boelter correlation can also predict the experimental HTCs outside the pseudocritical region, but deviates significantly from the experimental data within the pseudocritical region. It should be noted that both these correlations cannot be used for a prediction of HTCs within the deteriorated heat-transfer regime.

Experimental data of heat transfer coefficient during evaporation of R-1234yf, R-134a, and R-22 in horizontal circular small tubes are compared. The local heat transfer coefficient is obtained for heat fluxes ranging from 10 to 35 kW m −2 , mass fluxes ranging from 100 to 650 kg m −2 s −1 , saturation temperatures of 5, 10, and 15°C, and quality up to 1.0. The test sections are made of stainless steel tubes with inner diameters of 1.5 and 3.0 mm, the lengths of 1000 and 2000. Effects of heat flux, inner tube diameter, and saturation temperature on heat transfer coefficient are reported in the present study. Nucleate boiling heat transfer contribution is predominant, especially at low quality region, and laminar flow appears in the evaporative small tubes. The experimental results are compared against four existing heat transfer coefficients, and the modified correlation of heat transfer coefficient is developed with good prediction.

Flow pattern and heat transfer characteristics of R-134a refrigerant during flow boiling in a horizontal circular mini-channel

The effects of concentration of water‐based MgO nanofluid on heat transfer and pressure drop characteristics of a shell‐and‐tube heat exchanger is modelled and numerically simulated by using five kinds of volume concentrations of 0%, 0.2%, 0.35%, 0.4%, and 0.5%. Validations for modelling and algorithm are performed by experiments with respect to the parameters of average heat transfer and overall heat transfer coefficient by means of distilled water‐based medium, which are in well line with measurement data. The overall heat transfer coefficient, Nusselt number, pressure drop, friction factor, Prandtl number and performance evaluation criteria are predicted using commercial software Fluent 2020R2. Results showed that an increase in volume concentration of nanofluid enhance the heat transfer performances. Compared to water‐based medium, the peak values of the overall heat transfer coefficient and the Nusselt number are 14.52% and 13.02%, referring to the threshold of volume concentration of 0.35%.

A numerical model for the flow condensation of the binary zeotropic mixture

Bubble nucleation was investigated in a 20-mm-long, wickless heat pipe on the International Space Station. Over 20 h of running experiments using pentane as the working fluid, more than 100 nucleation events were observed. Bubble nucleation at the heater end temporarily boosted peak pressures and vapor temperatures in the device. At the moment of nucleation, the heater wall temperature significantly decreased due to increased evaporation and the original vapor bubble collapsed due to increased pressure. A thermal model was developed and using the measured temperatures and pressures, heat transfer coefficients near the heater end of the system were extracted. Peak heat transfer coefficients during the nucleation event were over a factor of three higher than at steady-state. The heat transfer coefficient data were all collapsed in the form of a single, linear correlation relating the Nusselt number to the Ohnesorge number.

Determination of operational flow regime and heat transfer performance optimization of mono and hybrid nanofluids using FOM and sensitivity analysis

ABSTRACTThe heat transfer coefficients of homogeneous and hybrid micro-porous copper foams, produced by the Lost Carbonate Sintering (LCS) process, were measured under one-dimensional forced convection conditions using water coolant. In general, increasing the water flow rate led to an increase in the heat transfer coefficients. For homogeneous samples, the optimum heat transfer performance was observed for samples with 60% porosity. Different trends in the heat transfer coefficients were found in samples with hybrid structures. Firstly, for horizontal bilayer structures, placing the high porosity layer by the heater gave a higher heat transfer coefficient than the other way round. Secondly, for integrated vertical bilayer structures, having the high porosity layer by the water inlet gave a better heat transfer performance. Lastly, for segmented vertical bilayer samples, having the low porosity layer by the water inlet offered the greatest heat transfer coefficient overall, which is five times higher than its homogeneous counterpart.

In the paper, the authors analyzed the solution of the differential equation of non-stationary heat conduction for an unbounded plate during the heat exchange of plate surfaces with the surrounding medium according to Newton’s law at a constant temperature of the medium. To use the results of solving the equations in the drying of thin flat materials, the dependence of the heat transfer coefficients on temperature and moisture content was studied. As a result of studying and analyzing a number of literature sources, the regularities of the change in the heat transfer coefficients during drying are established with high reliability. Studies of drying of thin wet plates of white and red clays with known heat transfer coefficients have shown that for small values of the heat transfer criterion of the Bio and small temperature gradients over the section of a thin material, application of the results of solutions of the heat transfer equations gives completely satisfactory agreement between the calculated and experimental values of the temperatures and the duration of drying. It is established that for small Bio numbers, the main factor is the external heat and mass transfer of the surface of the material with the surrounding medium and the rate of drying depends little on internal mass transfer. It is shown that the use of numerical methods for solving differential equations is possible with varying degrees of approximation only for accurate and reliable dependences of heat and mass transfer coefficients on moisture content and temperature. For a number of materials with known heat transfer coefficients, the use of analytical methods in calculations is of considerable interest and brings the theory closer to the practice of drying.

Application of nanofluid and coiled tubes are two passive methods for increasing the heat transfer. In the present study, the turbulent flows of water and nanofluid in coiled tubes heat exchanger were numerically studied. CuO-water nanofluid containing 1 vol% copper oxide nanoparticles was used and single-phase approach was considered for nanofluid flow. The effect of different geometrical parameters on Nusselt number was also investigated for both helical and conical coiled tubes. Water properties were defined as temperature dependent function, and constant wall temperature was employed as wall boundary condition. Simulation results were validated by available experimental and numerical data of heat transfer coefficient and pressure drop inside the helically coiled tube. The results show that for the helically coiled tubes, Nusselt number increased by tube diameter enhancement, while the increase of pitch circle diameter resulted in its decline. However, the variation of helical pitch exhibited a slight effect on the heat transfer. Moreover, for conical coiled tubes, increase helical pitch and cone angle reduced Nusselt number; while this parameter increased as the tube diameter increases. Addition of 1% copper oxide to water led to 8% heat transfer augmentation in the helically coiled tube. Also, using CuO-water nanofluid, the performance index was enhanced by 3.5 and 5% for helical and conical coiled tubes, respectively.

The heat transfer tube is widely regarded as the most critical component in heat exchangers, playing a pivotal role in determining the overall efficiency of thermal systems. Enhancing the pool boiling heat transfer performance of these tubes is, therefore, of utmost importance for improving the overall heat exchange process. In this study, to achieve enhanced thermal performance, two types of bilaterally enhanced tubes (designated as E1 and E2 tubes) with different surface structures were designed and fabricated. These tubes were specifically engineered to improve the heat transfer characteristics under pool boiling conditions. The design of these tubes incorporates a threaded structure on the inner surface, which serves to disrupt the boundary layer and enhance the fluid mixing inside the tube. Meanwhile, the outer surface of the tubes is equipped with a complex array of fins, designed to significantly increase the surface area available for heat transfer. To evaluate the effectiveness of these enhancements, heat transfer experiments were conducted on both types of bilaterally enhanced tubes under pool boiling conditions. Additionally, the experimental data obtained from these enhanced tubes were compared with that of a smooth tube, which was made of the same material and had the same diameter, in order to assess the performance improvements. In the experiments, R134a was used as the working fluid, and the tests were performed under saturated temperature conditions of 6 °C. The refrigerant R134a flowed on the external surface of the heat exchange tubes, while deionized water circulated inside the tubes. Under the condition of constant heat flux, the heat flux density was maintained at 30 kW/m2, and the water-side flow velocity ranged from 1 to 3 m/s. Under the condition of constant water-side inlet temperature, the water inlet temperature was controlled at 12°C. The results show that the overall heat transfer coefficient and boiling heat transfer coefficient on the refrigerant side of the bilaterally enhanced tubes can reach up to five times that of the smooth tube. The complex fin array on the outer surface of the enhanced tubes significantly increased the effective heat transfer area and the number of vaporization cores, promoting bubble growth and timely detachment, which in turn facilitated efficient heat transfer. Three different heat transfer coefficient correlations were selected to predict the heat transfer coefficient for the smooth tube. The most accurate boiling heat transfer coefficient correlation was identified and then modified using the experimental data from the enhanced tubes in this study. This resulted in a new predictive correlation for the boiling heat transfer coefficient applicable to the novel bilaterally enhanced tubes studied here. The modified correlation can predict over 90% of the heat transfer coefficient data points within an error range of ±20%.

The effect of Dixon rings on direct contact heat transfer performance: Comparison of counter and co-current evaporation

The relationship between ratio of length to diameter (L/D) of shell and tube heat exchangers (STHXs) and deep heat transfer was studied experimentally. The comparison of heat transfer and pressure drop performance of five STHXs with tube number of 10, 20, 30, 40, 50 are presented in this paper. The experiment is carried out in STHXs with cold air in shell side and hot air in tube side. Overall heat transfer coefficients are calculated and heat transfer coefficients of shell side are determined by Wilson plots technique. It shows that heat transfer performance decrease and pressure drop increase with the decrease of L/D, and critical point of deep heat transfer state (hot air outlet temperature equals to that of cold one) is achieved when L/D at some point between 3.08 and 4.62. When a certain heat transfer task is given, reasonable L/D can be chosen based on the experimental results.Keywordspressure dropfriction factorflow distributionratio of length to diameterdeep heat transfer

This paper presents a series of experiments on flow boiling heat transfer coefficient and pressure drop using refrigerant R32. Tests were performed in a smooth horizontal stainless steel tube with an internal diameter of 6.00 mm, with the heat directly applied on the test section tube through DC current. Trends of the heat transfer coefficient and pressure drop data are displayed for different operating conditions. Specifically, the saturation temperature was fixed to 25 and 35 °C, the heat flux varied from 5 to 50 kW/m2 and the mass velocity was fixed to the low values of 150 and 250 kg/m2 s. The effects of the operating parameters on the heat transfer results are also shown.

To accurately model warm and hot forming operations, correct values for heat transfer coefficient (HTC) are required, among other parameters. In this work, an experimental methodology is proposed to obtain HTC data for a high strength steel alloys (Usibor® 1500 A-S), seven aluminum alloys (AA5182-O, AA5754-O, AA6013-T6, AA6063-T6, AA6061-T6, AA7075-T6 and a developmental AA7xxx-T76) and two magnesium (AZ31B-O and ZEK100-O) alloys. The HTC was characterized as a function of contact pressure from 2 to 80 MPa. Overall, the HTC values exhibited significant variation (ranging from 355 to 1887 W/m2-K), but tended to correlate primarily with contact pressure and material strength/alloy system. Two lubricants were also considered (Fuchs Forge Ease 278 and PTFE spray) for some alloys, but had only a minor effect on HTC relative to “dry” tooling. Neither the initial temperature of the tooling nor the ramping up of the press tonnage had a noticeable effect on the HTC since the HTC values were identified based only upon the relative temperature difference and thermal properties of the materials. The experimentally determined HTC values were then applied in a series of non-isothermal cylindrical cup warm deep drawing experiments. The deep draws were modelled using finite element (FE) techniques, which utilized the measured HTC values. The temperature-time profiles from the deep draws were found to agree well with the predicted profiles, indicating that the methodology used to determine HTC and the resulting HTC values, are applicable for use in a metal forming simulation involving heat transfer.