Convective Boiling Heat Transfer Coefficients Research Articles

On the scaling of convective boiling heat transfer coefficient

Flow boiling has been widely used during the last century, but the key mechanisms controlling the heat transfer process still remain elusive. In the particular case of convective flow boiling inside a heated pipe, most of the existing correlations have been proposed assuming an enhancement factor to the liquid Reynolds number. In this work, we show that during convective boiling heat transfer, the vapor Reynolds number plays a dominant role which has been overlooked. By comparing experimental data in this work and from the literature, we show that the enhancement factor to the liquid Reynolds number does not appropriately correlate the heat transfer coefficient. Further, it can be shown that most of the proposed enhancement factors can be re-written in terms of the explicit contribution of the ReV. In particular, at high qualities, the influence of ReL becomes negligible.

Phenomenological correlations for two phase heat transfer and pressure drop performance of electrohydrodynamic horizontal convective boiling of dielectric fluids

EHD is an active heat transfer enhancement technique for convective boiling or condensing dielectric fluids whose performance varies widely in the literature for even the same geometry and fluids. As such, the performance of EHD convective boiling devices remains largely unpredictable. Two empirical EHD convective boiling heat transfer coefficient correlations exist in the literature with good performance when compared to their exact test dataset and geometry, however they have been shown to have poor correlations in predicting the performance for external test datasets. In this paper, a phenomenological approach is taken in the development of a new EHD convective boiling heat transfer coefficient correlation and a novel EHD convective boiling pressure drop correlation. The performance correlations are based on widely-used free-field convective boiling correlations for heat transfer coefficient and pressure drop with phenomenological enhancement factors based on the two phase flow pattern as predicted using the EHD two phase flow pattern map which accounts for the effect of electric field strength on flow pattern redistribution, as described in a previously published paper (Nangle-Smith and Cotton, 2018). A review of the EHD convective boiling experimental literature was conducted to determine confounding factors that can impact the large variance in the reported performance. Flow pattern and applied heat flux were identified as common parameters not maintained constant in the experimental data. Two datasets, including data from the present study, in which flow pattern and applied heat flux are maintained constant were used in the development of the performance correlations. Considerable improvement over previous EHD convective boiling performance correlations was found both in error and the physical significance of coefficients. The correlations were developed for thermodynamic qualities in the range of 20–60% below the onset of dryout or mist flow patterns and for applied heat fluxes < 30 kW/m2. Furthermore, test conditions in this study were chosen to focus on the dielectrophoretic effect and therefore studies with significant electrophoresis were not included. Thus, the correlation is limited to positive applied voltages, saturated flow boiling conditions, and field strengths below the onset of charge injection. The authors recommend this approach for EHD two phase performance correlation development as it is more mechanistic, is analogous to the state-of-the-art approaches for free-field two phase performance, and is likely to yield more accurate models as more experimental data becomes available.

Experimental investigation of boiling heat transfer in helically coiled tubes at high pressure

Helically coiled once-through steam generators have been used widely during the past several decades for small nuclear power reactors. The boiling heat transfer characteristics of helically coiled tubes are important to optimal design of helically coiled steam generators. In the present work, various experiments with helically coiled tubes are performed to investigate the boiling heat transfer characteristics at high pressure with water as the working medium. Six test sections are made of thin walled stainless steel tubes having inner diameters of 12.5 mm and 14.5 mm. The system pressure is varying from 2 to 8 MP. The effects of heat flux, mass flux and system pressure on boiling heat transfer behavior are discussed in detail. Increase in heat flux increases the subcooled boiling and saturated nucleate boiling heat transfer coefficient. However, heat flux has no influence on saturated convective boiling heat transfer coefficient. Mass flux increases the saturated convective boiling heat transfer coefficient. Increase in system pressure increases the subcooled boiling and saturated nucleate boiling heat transfer coefficient but decreases the saturated convective boiling heat transfer coefficient. For the investigated experimental conditions, the results show that the Baburajan's correlation gives the best prediction in subcooled boiling region and the Gungor and Winterton's correlation predicts well with the experimental data in saturated boiling region. For the investigated experimental conditions, the results also indicate that the helically coiled tubes have similar performance of heat transfer in boiling regions compared with straight tubes.

공조용 알루미늄 납작관 내의 R-410A 대류 비등

Abstract Flat tube heat exchangers can improve the thermal performance significantly compared with round tube heatexchangers. For proper design of flat tube heat exchangers, one should know the tubeside heat transfer coefficients.In this study, convective boiling heat transfer coefficients of R-410A were obtained in a flat extruded aluminum tubewith D h =1.41 ㎜ . The test range covered mass flux from 200 to 600kg/m 2 s, heat flux from 5 to 15kW/m 2 and saturationtemperature from 5 ℃ to 15 ℃ . The heat transfer coefficient curve shows a decreasing trend after a certain quality(critical quality). The critical quality decreases as the heat flux increases, and as the mass flux decreases. Theearly dryout at a high heat flux results in a unique 'cross-over' of the heat transfer coefficient curves. The heat transfercoefficient increases as the mass flux increases. At a low quality region, however, the effect of mass flux is not prominent. The heat transfer coefficient increases as the saturation temperature increases. The effect of saturation temperature, however, diminishes as the heat flux decreases. Both the Shah and the Kaew-On et al. correlations reasonably predicted the present data.

Experimental investigation of the forced convective boiling heat transfer of R-600a/oil/nanoparticle

In this paper an experimental study of convective boiling heat transfer of R-600a/oil/nanoparticle mixtures is investigated. The experimental setup was prepared with a smooth horizontal tube as a test section with the length and diameter of 9.5 and 103mm, respectively, and pure R-600a was applied for evaluating the heat transfer enhancement. Six mixtures containing 1% weight fraction of R-600a/oil with different concentrations of CuO nanoparticles including 0.0, 0.5, 1.0, 1.5, 2.0 and 5.0% weight fraction of R-600a/oil/nanoparticle were used in our study.The mass velocity per cross area was considered at the range of 50–700kg/m2s for low vapor quality (ϕ<0.25). The results showed that the convective boiling heat transfer coefficient will be increased by increasing the mass fraction of nanoparticles up to 2%, while by increasing the mass fraction of nanoparticles up to 5% the heat transfer coefficient will be reduced.

Sedimentation and convective boiling heat transfer of CuO-water/ethylene glycol nanofluids

The convective boiling characteristics of dilute dispersions of CuO nanoparticles in water/ethylene glycol as a base fluid were studied at different operating conditions of (heat fluxes up to 174 kW m−2, mass fluxes range of 353–1,059 kg m−2 s−1 and sub-cooling level of 343, 353 and 363 K) inside the annular duct. The convective boiling heat transfer coefficients of nanofluids in different concentrations (vol%) of nanoparticles (0.5, 1, and 1.5) were also experimentally quantified. Results demonstrated the significant augmentation of heat transfer coefficient inside the region with forced convection dominant mechanism and deterioration of heat transfer coefficient in region with nucleate boiling dominant heat transfer mechanism. Due to the scale formation around the heating section, fouling resistance was also experimentally measured. Experimental data showed that with increasing the heat and mass fluxes, the heat transfer coefficient and fouling resistance dramatically increase and rate of bubble formation clearly increases. Obtained results were then compared to some well-known correlations. Results of these comparisons demonstrated that experimental results represent the good agreement with those of obtained by the correlations. Consequently, Chen correlation is recommended for estimating the convective flow boiling heat transfer coefficient of dilute CuO-water/ethylene glycol based nanofluids.

Direct estimation of local convective boiling heat transfer coefficient in mini-channel by using conjugated gradient method with adjoint equation

The purpose of this paper was to present an inverse heat conduction method used for determining the local convective boiling heat transfer coefficient in mini-channel for pure water, copper nanofluid by using three different concentrations of nanoparticles: 5mg/L, 10mg/L and 50mg/L. Conjugated gradient method with adjoint equation is used to solve the IHCP and estimate directly the space-variable convective heat transfer coefficient. Direct estimation local convective boiling heat transfer coefficient is a nonlinear inverse heat problem. The uncertainties in the estimated in heat transfer coefficient are calculated using bias and variance errors. This method is able to estimate local convective boiling heat transfer coefficient very well.

Boiling heat transfer of co- and counter-current microchannel heat exchangers with gas heating

In this paper, we present an experimental study on a two-phase boiling heat transfer of liquid methanol in co- and counter-current microchannel heat exchangers (MCHEs) with gas heating. The silicon-based MCHE with dimensions of 20×20×2mm is designed with 18 microchannels. The working fluids on the hot and cold sides of the MCHE are helium and liquid methanol, respectively. During the study, the helium fluid inlet temperature is varied from 40 to 250°C, while the mass flux is fixed at 10kg/m2s; additionally, the mass flux of the liquid methanol is varied from 3 to 9kg/m2s, and the inlet temperature is varied from room temperature to the saturation temperature of methanol, depending on the hot-side heat flux. The results demonstrate that the convective boiling heat transfer increases with the mean vapor quality until approaching the critical heat flux (CHF) condition. The counter-current MCHE exhibits a higher CHF due to a higher inlet subcooling temperature than the co-current MCHE. The single-phase and convective boiling heat transfer coefficients are measured and compared with the existing correlations. Moreover, two empirical correlations for the two-phase boiling heat transfer coefficient for the MCHEs are proposed in this study.

Estimation local convective boiling heat transfer coefficient in mini channel

The aim of this paper is to present an inverse heat conduction method used for determining the local convective boiling heat transfer coefficient in mini channel for pure water, copper nanofluid with using three different concentrations of nanoparticles: 5mg/L, 10mg/L and 50mg/L. Sequential specification function method is used to solve the IHCP and estimate the space-variable convective heat transfer coefficient. The uncertainties in the estimated in heat transfer coefficient are calculated using Bias and Variance errors. The technique is used in a series of numerical experiments to provide the optimum experimental design for a boiling heat transfer investigation.

Algebraic turbulence modeling in adiabatic and evaporating annular two-phase flow

The study considers algebraic turbulence modeling in adiabatic and evaporating annular two-phase flow, focusing in particular on momentum and heat transfer (so-called ‘convective boiling’) through the annular liquid film. In contrast with single-phase wall-bounded flow theory, shear-driven annular liquid films are assumed here to behave as fluid-bounded flows, mostly interacting with the shearing gas-entrained droplets core flow. Besides providing velocity and temperature profiles through the liquid film, the turbulence model proposed here predicts key parameters such as the average liquid film thickness, the void fraction and the convective boiling heat transfer coefficient with accuracies comparable or better than those of leading design correlations. This turbulence model is part of a unified annular flow modeling suite that includes methods to predict the entrained liquid fraction and the axial frictional pressure gradient. The underlying heat transfer database covers nine fluids (water, two hydrocarbons and six refrigerants) for vertical and horizontal tubes of 1.03–14.4 mm diameter and pressures of 0.1–7.2 MPa. Importantly, this study shows that there appears to be no macro-to-microscale transition when it comes to annular flow. Simply better physical modeling is required to span this range.

Flow Boiling Heat Transfer in Microchannels

Flow boiling heat transfer to water in microchannels is experimentally investigated. The dimensions of the microchannels considered are 275×636 and 406×1063μm2. The experiments are conducted at inlet water temperatures in the range of 67–95°C and mass fluxes of 221–1283kg∕m2s. The maximum heat flux investigated in the tests is 129W∕cm2 and the maximum exit quality is 0.2. Convective boiling heat transfer coefficients are measured and compared to predictions from existing correlations for larger channels. While an existing correlation was found to provide satisfactory prediction of the heat transfer coefficient in subcooled boiling in microchannels, saturated boiling was not well predicted by the correlations for macrochannels. A new superposition model is developed to correlate the heat transfer data in the saturated boiling regime in microchannel flows. In this model, specific features of flow boiling in microchannels are incorporated while deriving analytical solutions for the convection enhancement factor and nucleate boiling suppression factor. Good agreement with the experimental measurements indicates that this model is suitable for use in analyzing boiling heat transfer in microchannel flows.

2624 円管内における強制対流下での膜沸騰熱伝達に関する研究(S57-1 原子炉システムおよびその要素技術(1),S57 原子炉システムおよびその要素技術)

In this research, the practical correlation and flow regime model is proposed on the basis of heat transfer coefficient analyzed from boiling heat transfer experiment, under temperature control, using following experimental parameter: mass flux in upstream [kgm^<-2>s^<-1>] and sub-cooled temperature in upstream [K]. As the result, it is indicated that forced convective boiling heat transfer coefficient under high mass flux condition show higher value than that of pool boiling. Correlation for heat transfer coefficient and experimental value agree well in DNB region and is demonstrated adequacy and practicability. This flow regime model is presupposed as high concentration droplet flow. And it is also noted that the gravity and centrifugal force affect to the heat transfer coefficient in some experimental condition for horizontal straight or curve tube.

Evaporative heat transfer and pressure drop of R410A in microchannels

Convective boiling heat transfer coefficients and two-phase pressure drops of R410A are investigated in rectangular microchannels whose hydraulic diameters are 1.36 and 1.44 mm. The mass flux was varied from 200 to 400 kg/m 2s, heat flux from 10 to 20 kW/m 2, as the saturation temperatures were maintained at 0, 5 and 10 °C. A direct heating method was used to provide heat flux into the fluid. The boiling heat transfer coefficients of R410A in the microchannels were much different with those in single tubes, and the test conditions only slightly affected the heat transfer coefficients before dryout vapor quality. The present heat transfer correlation for microchannels, which was developed by introducing non-dimensional parameters of Bo, We l, and Re l used in the existing heat transfer correlations for large diameter tubes, yielded satisfactory predictions of the present data with a mean deviation of 18%. The pressure drops of R410A in the microchannels showed very similar trends with those in large diameter tubes. The existing two-phase pressure drop correlations for R410A in microchannels satisfactorily predicted the present data.

Convective boiling heat transfer characteristics of CO 2 in microchannels

Convective boiling heat transfer coefficients and dryout phenomena of CO 2 are investigated in rectangular microchannels whose hydraulic diameters range from 1.08 to 1.54 mm. The tests are conducted by varying the mass flux of CO 2 from 200 to 400 kg/m 2 s, heat flux from 10 to 20 kW/m 2, while maintaining saturation temperature at 0, 5 and 10 °C. Test results show that the average heat transfer coefficient of CO 2 is 53% higher than that of R134a. The effects of heat flux on the heat transfer coefficient are much significant than those of mass flux. As the mass flux increases, dryout becomes more pronounced. As the hydraulic diameter decreases from 1.54 to 1.27 mm and from 1.27 to 1.08 mm at a heat flux of 15 kW/m 2 and a mass flux of 300 kg/m 2 s, the heat transfer coefficients increase by 5% and 31%, respectively. Based on the comparison of the data from the existing models with the present data, the Cooper model and the Gorenflo model yield relatively good predictions of the measured data with mean deviations between predicted and measured data of 21.7% and 21.2%, respectively.

Prediction of forced convective boiling heat transfer coefficient of pure refrigerants and binary refrigerant mixtures inside a horizontal tube

Forced convective boiling heat transfer coefficients were predicted for an annular flow inside a horizontal tube for pure refrigerants and nonazeotropic binary refrigerant mixtures. The heat transfer coefficients were calculated based on the turbulent temperature profile in liquid film and vapor core considering the composition difference in vapor and liquid phases, and the nonlinearity in mixing rules for the calculation of mixture properties. The heat transfer coefficients of pure refrigerants were estimated within a standard deviation of 14% compared with available experimental data. For nonazeotropic binary refrigerant mixtures, prediction of the heat transfer coefficients was made with a standard deviation of 18%. The heat transfer coefficients of refrigerant mixtures were lower than linearly interpolated values calculated from the heat transfer coefficients of pure refrigerants. This degradation was represented by several factors such as the difference between the liquid and the overall compositions, the conductivity ratio and the viscosity ratio of both components in refrigerant mixtures. The temperature change due to the concentration gradient was a major factor for the heat transfer degradation and the mass flux itself at the interface had a minor effect.

An Experimental Study of Convective Boiling in a Compact Serrated Plate-Fin Heat Exchanger

The experimental apparatus used to study boiling of a pure refrigerant (propane) during a vertical upflow inside a compact serrated plate-fin exchanger is presented in this paper. The experimental conditions perfectly reflect those occurring in industrial applications. The maximal heat rate exchanged between both fluids is equal to 70 kW. The propane evaporates in the exchanger and the outlet vapour quality is between 0.15 and 0.9. Tests are carried out with propane mass fluxes between 12 and 71 kg m m 2 s m 1 for pressures between 0.46 and 1.17 MPa. From the measurements carried out on the loop, the data processing allows us to deduce the propane local boiling heat transfer coefficients. An analysis of the measured convective boiling heat transfer coefficients, without nucleate boiling, shows the separate effects of quality, mass flux, and pressure. The trends observed have been identified as characteristic of slug flow. They are very different from those of the annular flow regime.

Forced convective boiling of pure refrigerants in a bundle of enhanced tubes having pores and connecting gaps

In this study, convective boiling tests were conducted for enhanced tube bundles. The surface geometry consists of pores and connecting gaps. Tubes with three different pore sizes ( d p=0.20, 0.23 and 0.27 mm) were tested using R-123 and R-134a for the following range: 8 kg/ m 2 s⩽G⩽26 kg/ m 2 s , 10 kW/ m 2⩽q⩽40 kW/ m 2 and 0.1⩽ x⩽0.9. The convective boiling heat transfer coefficients were strongly dependent on heat flux with negligible dependency on mass flux or quality. For the present enhanced geometry (pores and gaps), the convective effect was apparent. The gaps of the present tubes may have served routes for the passage of two-phase mixtures, and enhanced the boiling heat transfer. The convective effect was more pronounced at a higher saturation temperature. More bubbles will be generated at a higher saturation temperature, which will lead to enhanced convective contribution. The pore size where the maximum heat transfer coefficient was obtained was larger for R-134a ( d p =0.27 mm ) compared with that for R-123 ( d p =0.23 mm ). This trend was consistent with the previous pool boiling results. For the enhanced tube bundles, the convective effect was more pronounced for R-134a than for R-123. This trend was reversed for the smooth tube bundle. Possible reasoning is provided based on the bubble behavior on the tube wall. Both the modified Chen and the asymptotic model predicted the present data reasonably well. The RMSEs were 14.3% for the modified Chen model and 12.7% for the asymptotic model.

Experimental Study on Closed-Loop Two-Phase Thermosyphon Devices for Cooling MCMs

Thermosyphon cooling modules, to cool multichip modules (MCMs), were designed and tested. The cooling module consists of a cold plate with microfinned channels and a plate-type integrated condenser. A separate flow model was employed to predict the mass flux and the pressure drop in the channel of the cold plate. The local and average convective boiling heat transfer coefficients and the corresponding wall superheat were calculated using the Chen's correlation. Experiments were performed to find out how the thermal performance of the cooling module was affected by the condenser size and the amount of charging fluid. Great emphasis was placed on the transient characteristics of the cooling module. For an allowable temperature rise of 58 ° C on the surface of the heater, the cooling module can handle a heat flux of as much as 2.5 W/cm2. No boiling retardation was observed inside the cold plate, which resulted in smooth transition from the transient state to the steady one. It was also found that the appropriate size of the condenser and the adequate amount of charging liquid are crucial factors affecting the performance of a closed two-phase thermosyphon device.

Forced Convective Boiling of Pure Refrigerants in a Bundle of Enhanced Tubes

In this study, convective boiling tests were conducted for enhanced tube bundles. The surface geometry consists of pores and connecting gaps. Tubes with three different pore sizes (d = 0.20, 0.23 and 0.27 mm) were tested using R-123 and R-l34a for the following range: 8 kg/ms G 26 kg/ms, 10 kW/m q0 40 kW/mand 0.1 0.9. The convective boiling heat transfer coefficients were strongly dependent on heat flux with negligible dependency on mass flux or quality. For the present enhanced geometry (pores and gaps), the convective effect was apparent. The gaps of the present tubes may have served routes for the passage of two-phase mixtures, and enhanced the boiling heat transfer. The convective effect was more pronounced at a higher saturation temperature. More bubbles will be generated at a higher saturation temperature, which will lead to enhanced convective contribution. The pore size where the maximum heat transfer coefficient was obtained was larger for R-l34a (d = 0.27 mm) compared with that for R-123 (d = 0.23 mm). This trend was consistent with the previous pool boiling results. For the enhanced tube bundles, the convective effect was more pronounced for R-134a than for R-123. This trend was reversed for the smooth tube bundle. Possible reasoning is provided based on the bubble behavior on the tube wall. Both the modified Chen and the asymptotic model predicted the present data reasonably well. The RMSEs were 14.3% for the modified Chen model and 12.7% for the asymptotic model.model.

Boiling heat transfer of a ternary refrigerant mixture inside a horizontal smooth tube

Abstract A theoretical model for predicting the forced convective boiling heat transfer coefficients of ternary mixtures is presented. The effect of mass diffusion near the gas-liquid interface is considered in both nucleate boiling and forced convection vaporization contributions. The predicted heat transfer coefficients are compared with experimental data, which are obtained for an HFC32: 125: 134a (23: 25: 52 wt%) mixture inside a horizontal smooth tube 6.0 mm i.d. and 4.0 m long, and in the ranges of heat flux 10–20 kW m−2 and of mass flux 150–400 kg m−2 s−1. The present predictive method shows reasonable agreement with the experimental data within an accuracy of ±30%.