Non-boiling Region Research Articles

Numerical investigation of the effect of vibration on spray cooling heat transfer in the non-boiling region

Vibrations profoundly influence the liquid film formed by spray droplets in spray cooling applications. However, understanding heat transfer mechanisms in the non-boiling region under vibrating conditions is limited in the existing literature. This study aims to bridge this gap by numerically investigating the heat transfer performance of spray cooling in non-boiling regions under vibration using the Lagrange-Euler method. To validate the computational fluid dynamics (CFD) model, experimental measurements of spray heat transfer were conducted on a heated vibrating surface test rig. The results demonstrate that the average film thickness (AFT) and average film velocity (AFV) increase at lower frequencies, leading to a decrease in the heat transfer coefficient. Conversely, the AFT decreases at higher frequencies while the AFV remains relatively constant. This reduction in AFT enhances the heat transfer coefficient, resulting in an up to 12% improvement in heat transfer performance. When a larger vibration amplitude is introduced, a higher inertial force acts on the liquid film, causing it to converge towards the centre. Generally, the heat transfer coefficient of spray cooling in the non-boiling region decreases as the vibration amplitude increases.

FLOW BOILING HEAT TRANSFER AND HEAT CONDUCTION ANALYSIS OF HIGH-POWER ELECTRONIC DEVICES

A flow boiling experiment in a narrow channel was conducted using subcooled water. Moreover, the experimentally measured heat flux of the channel was applied for the heat conduction analysis of insulated gate bipolar transistors (IGBTs). The relationship between the heat flux and the temperature difference was investigated experimentally, ranging from nonboiling to boiling in the narrow tube. The boiling heat transfer in the fully nucleate boiling region was validated by comparing with the Rohsenow correlation. Moreover, the measured critical heat flux (CHF) agreed with the Hall and Mudawar correlation within ± 15%. The heat conduction analysis of an IGBT module with a heatsink was solved using the measured heat flux in the narrow channel as a boundary condition, applied from the nonboiling region to the film boiling region. The thermal resistance of the material was obtained when the heatsink with narrow channels was modeled. The numerical result indicated that the thermal resistance of the material was influenced by the heat transfer process of the cooling channel.

Experimental investigation on the characteristics of film thickness and temperature on the heated surface during spray cooling

The heat dissipation problem has become a new challenge for improving the comfort of the built environment. Spray cooling, as an efficient heat exchange technique, attracts more attention. In the present study, an experimental work was performed to investigate the influence of the liquid film produced by a pressure-swirl nozzle on the heat transfer in spray cooling. The film thickness and heated surface temperature at different water flow rates (100–160 ml/min), spray heights (2–10 mm) and temperature of heaters (0–140 °C) were studied. The experimental results indicated that the water film was not flat, and a raised zone was formed under the pressure-swirl nozzle. The time-average film thickness of the raised zone first increased and then decreased with the increase of spray heights. Besides, the amplitude of the water film became larger with the temperature of heaters, and the maximum thickness was 3.82 mm. The heat transfer under experimental conditions was located in the non-boiling region. The non-uniformity of the temperature distribution became more significant with the temperature of heaters, and the maximum standard deviation was 4.66 °C. Besides, the main heat transfer mechanism inside the raised zone was the droplet impingement instead of the raised zone’s collapse when spray heights were more than 6 mm, and the heat outside of the raised zone was mainly removed by the film convection. The experimental findings are conducive to further study on the heat transfer under spray cooling.

A mechanistic model for wall heat flux partitioning based on bubble dynamics during subcooled flow boiling

This paper aimed to develop a mechanistic model for wall heat flux partitioning based on bubble dynamics in horizontal subcooled flow boiling. The model was established from two dimensions of space and time. In the space dimension, the heating surface was divided into three regions based on the motion characteristics of bubbles, including bubble nucleation region, bubble sliding region, and non-boiling region. In the time dimension, the heat transfer mechanism on the heating surface of the bubble nucleation region and bubble sliding region could be divided into evaporation, condensation, and transient heat transfer. An evaporation model for subcooled boiling was developed based on condensation at the top of the bubble. A transient quench heat transfer model for subcooled flow boiling was modified. The heat flux of each region was obtained by the sum of the time-averaged heat fluxes of these heat transfer mechanisms during a bubble periodic time. The total wall heat flux was obtained by the sum of the area weights of the heat fluxes in these three regions. The new wall heat flux partitioning model and the RPI model were used to predict the experimental values at a variety of flow conditions. The results showed that the new model could well predict the experimental values and the deviation between the predicted heat flux and the experimental results was within 25%. The study also found that it was necessary to consider the condensation at the top of the bubble and the heat transfer mechanism caused by bubble sliding.

Conductive and viscous sub-layers on forced convection and mechanism of critical heat flux during flow boiling of subcooled water in a platinum circular tube with 3 mm inner diameter and 32.7 mm heated length at high liquid Reynolds number

The turbulent heat transfer, the subcooled boiling heat transfer and the steady state CHF for a Pt-circular test tube of a 3 mm inner diameter and a 32.7 mm heated length are measured with a wide range of inlet subcooling and flow velocity at high liquid Reynolds number, i.e. Red = 2.35 × 104 to 1.12 × 105. For flow velocities ranging from 4.103 to 21.446 m/s, the inner surface temperature of the Pt circular test tube calculated by the steady one-dimensional heat conduction equation is compared with the value obtained from our turbulent heat transfer correlation equation and the numerical solution of the RANS equation (Reynolds mean Navier–Stokes simulation) of the k-ε turbulence model. The conduction sub-layer thicknesses from the non-boiling region to CHF are measured in the conduction sub-layer itself in the forced convection region and the thinner sub-layer dissipated by boiling evaporation in the nucleate boiling region. The nondimensional thicknesses of local and average conductive sub-layers and, the thicknesses and nondimensional thicknesses of local and average viscous sub-layers on forced convection are estimated from the thicknesses of local and average conductive sub-layers and Prandtl numbers evaluated at the calculated temperature of the first control volume on the heated surface. In addition, the thickness of the conductive sublayer at the CHF point is estimated from measurements at various flow velocities. The experimental values of the CHF are also compared with authors’ widely and precisely predictable correlations of critical heat flux during flow boiling of subcooled water and the corresponding theoretical values of the liquid sub-layer dry-out models suggested by other researchers, respectively. The authors’ correlations and other researchers’ theoretical values can represent the subcooled boiling CHFs obtained in this study within the ranges of −22.74 to −6.21% difference and −22.17 to 6.16% one, respectively. Proposals for the main mechanism of critical heat flux during flow boiling of subcooled water on vertical tubes are confirmed again at high liquid Reynolds numbers based on experimental data. The boiling transitions to film boiling at the subcooled water flow boiling in the Pt test tube of d = 3 mm and L = 32.7 mm is not due to heterogeneous spontaneous nucleation or hydro-dynamic instability, but to liquid sub-layer dry-out model at the steady-state CHF. It is similar to those for the Pt test tubes with d = 3 mm and L = 66.5 mm, and d = 3 mm and L = 100 mm.

Experimental and numerical study of air-water mist jet impingement cooling on a cylinder

The paper presents the study of experimental as well as the numerical study of air-water mist jet impingement cooling over a heated cylinder in the non-boiling region. The studies are conducted for various mist loading fraction, f = 0.0025, 0.0050, 0.0075 and 0.01; Reynolds number, Remix.=8000, 10,500, 13,000 and 15,500; and surface-to-nozzle spacing, H/d = 30, 40, 50 and 60. Enhancement in the heat transfer (η) for mist jet impingement as compared to air jet impingement is obtained from experimental and numerical analysis. The numerical study also helped in understanding the distribution and tracking of droplets in the computational domain. High enhancement in the heat transfer is observed with high mist loading fraction. Also, the lowest Reynolds number yielded the highest enhancement in heat transfer with comparison to the higher Reynolds number. At various surface-to-nozzle spacing, the enhancement in heat transfer is high for low surface-to-nozzle spacing near the stagnation zone to a certain point, after which point the enhancement reverses as higher enhancement in heat transfer is observed for high surface-to-nozzle spacing. As high as 408% and 775% enhancement in the heat transfer at the stagnation point is observed for f = 0.01, Remix.= 8000and H/d = 30 during experimental and numerical analysis respectively. The correlation has been proposed to estimate the enhancement in heat transfer at the stagnation point.

Experimental Investigation on Heat Transfer Mechanism of Air-Blast-Spray-Cooling System with a Two-Phase Ejector Loop for Aeronautical Application

This paper presents an air-oriented spray cooling system (SCS) integrated with a two-phase ejector for the thermal management system. Considering its aeronautical application, the spray nozzle in the SCS is an air-blast one. Heat transfer performance (HTP) of air-water spray cooling was studied experimentally on the basis of the ground-based test. Factors including pressure difference between water-inlet-pressure (WIP) and spray cavity one (PDWIC) and the spray volumetric flow rate (SVFR) were investigated and discussed. Under a constant operating condition, the cooling capacity can be promoted by the growth factors of the PDWIC and SVFR with the values from 51.90 kPa to 235.35 kPa and 3.91 L ⋅ h − 1 to 14.53 L ⋅ h − 1 , respectively. Under the same heating power, HTP is proportional to the two dimensionless parameters Reynolds number and Weber number due to the growth of droplet-impacting velocity and droplet size as the increasing of PDWIC or SVFR. Additionally, compared with the factor of the droplet size, the HTP is more sensitive to the variation in the droplet-impacting velocity. Based on the experimental data, an empirical experimental correlation for the prediction of the dimensionless parameter Nusselt number in the non-boiling region with the relative error of only ± 10 % was obtained based on the least square method.

Numerical simulation and experimental investigation on spray cooling in the non-boiling region

In this study, numerical simulation and experimental investigation were conducted with distilled water as the test liquid to study the spray cooling heat transfer characteristics in the non-boiling region. The Euler-Lagrangian approach was employed to capture the transient behavior of the dispersed phase while the continuous phase is considered in a steady state. The Realizable k-e model was also adopted to account for the turbulence effect during spray cooling process. The effect of droplet diameter on spray cooling heat transfer was investigated. The comparison of wall temperature between simulation result and experimental data demonstrates a favorable accordance with a maximum deviation below 2.5%. The results show that droplet diameter is an important influence factor on spray cooling characteristics in the non-boiling region, especially under low heat flux condition. A smaller droplet size will result in a thicker wall film and a higher heat transfer coefficient under the same heat flux, consequently a lower wall temperature can be obtained.

Gas-assisted evaporation and boiling in minichannels

This study experimentally explores the heat transfer characteristics of gas-assisted evaporation and boiling in single or two parallel minichannels under both non-boiling and boiling conditions. The liquid working fluid used is ethanol and the inert gas is helium. Compared to the pure ethanol flow in a minichannel under both non-boiling and boiling conditions, the heat transfer enhancement (HTE) caused by the adjunction of helium is examined. The maximal HTE owing to an inert gas is located at the wall superheat (ΔTsat) of −10°C (i.e., non-boiling region) in the single minichannel in which the annular flow occurring in most parts of the channel; however, for the boiling region, the HTE is insignificant in the single minichannel, as the flow patterns observed for the studied cases are approximately the same. For the two parallel minichannels, the differences in the mean (effective) wall heat flux between the cases with and without helium become much more evident than those in a single minichannel. Under boiling conditions, primarily an annular flow, accompanied with bubble nucleation at the wall downstream, occurs for the studied cases when helium flow is present; however, extensive bubble nucleation occurs and bubbly flow prevails for the case without helium flow. Owing to the difference in flow pattern, the heat transfer performances for the cases with and without helium are significantly different in the parallel minichannels under this boiling condition. In addition, the HTE significantly increases with an increase in the helium flow rate at a given wall superheat. The maximal HTE, which also occurred at about ΔTsat=−10°C, is 206% obtained under the conditions of the lowest ethanol flow rate and the highest helium flow rate in the parallel minichannels.

Experimental Investigation of Continual- and Intermittent-Spray Cooling

This work involved continual- and intermittent-spray cooling heat transfer experiments on a flat surface to study the effects of the spray cycle, duty ratio, and spray time. The spray droplet parameters were similar, while the heat transfer coefficients for continual-spray cooling were appreciably larger than those of intermittent-spray cooling for the same pressure; however, the heat dissipated per kilogram of water for the intermittent-spray cooling was larger, especially in the non-boiling region. The results show that an optimal spray cycle and optimal duty ratio make more efficient use of the coolant in intermittent-spray cooling.

Capability of RELAP5 MOD3.3 code to simulate density wave instability in parallel narrow rectangular channels

Density Wave Oscillation (DWO) is one of the most common types of instability in boiling systems. Many experiments were conducted to investigate DWO in various geometry channels during last several decades, but what we always want to know is the stability characteristic of one specific system. In this paper, DWO in narrow rectangular channels which have a cross section of 25mm×2mm and a heated length of 1000mm are investigated by means of RELAP5/MOD3.3. Then 62 DWO experimental data in two narrow rectangular channels and 60 DWO experimental data in two narrow rectangular channels with a large bypass over 1–10MPa of pressure, 200–800kg/m2s of mass velocity, and 10–50°C of inlet subcooling conditions are adopted to verify the RELAP5 results. The thermal hydraulic behavior, parametric effect study, and flow instability boundary in narrow rectangular channels are simulated and compared with the experimental results. The errors associated with the predictions are addressed. The density, flow rate and pressure drop characteristics during DWO condition are also analyzed. Compared with experimental flow instability boundary, 90% prediction data for two narrow rectangular channels are between ±20% errors and 85% data for two narrow rectangular channels with a large bypass are between ±20% errors. The error becomes higher under the low pressure with high subcooling, low flow rate condition and high pressure condition. During DWO condition, density, flow rate and pressure drop of the heated channels oscillate periodically; flow rates are different along the channel and the inlet and outlet flow rates oscillate almost in reverse and the non-boiling region pressure drop oscillates out-of-phase with respect to boiling region pressure drop. The results aim to be a contribution to the assessment of the code capability to detect the onset of DWO in narrow rectangular channels. The purpose of the paper is to highlight strengths and weaknesses of the code to simulate DWOs as well as to evaluate carefully the suitable numerical settings necessary to assure a correct prediction of the phenomenon.

Forced convection heat transfer of subcooled liquid nitrogen in a vertical tube

Experimental research on forced convection heat transfer of subcooled liquid nitrogen ranging from the pressures of 0.3 MPa to its supercritical pressure is carried out for wide ranges of inlet temperature and flow velocity. A stainless steel tube heater with the inner diameter of 5.4 mm and the length of 100 mm is mounted vertically. The heat transfer coefficients in non-boiling region and the DNB (departure from nucleate boiling) heat fluxes are higher for higher flow velocity and higher subcooling. The trend of the heat transfer coefficients in the non-boiling region agree with those by the Dittus-Boelter correlation, although they are unaffected by the flow velocity for Re < 8000. The lowest limits agree with those obtained for no forced flow. The correlation of DNB heat flux that can describe the experimental data is presented. On the other hand, forced convection heat transfer of supercritical nitrogen deteriorates when the heated surface temperature, Tw, exceeds its pseudo-critical temperature, Tc', although they are similar to those under subcritical pressure for Tw < Tc'. The heat transfer characteristics of supercritical nitrogen can be predicted by authors' correlation.

High heat load test of CFC divertor target plate with screw tube for JT-60 superconducting modification

A flat carbon fiber composite (CFC) tile mock-up with screw tubes, which have helical fins like a nut, was fabricated aiming at further improvement of the heat removal performance of the cost-effectively manufactured divert or target for JT-60SC (modified JT-60 as a superconducting coil tokamak). The heat removal performance of the mock-up was successfully demonstrated on the JAERI Electron Beam Irradiation Stand. The estimated heat transfer coefficient of the screw tube at the non-boiling region was roughly three times higher than that of the smooth tube. This corresponds to 1.5 times that of the swirl tube. A heat cycle test of 10 MW/m 2 showed that the mock-up with the screw tubes could withstand for 1400-cycles. These results indicate that the divertor target plate with the flat CFC tile and the screw tube can be a promising candidate for the JT-60SC divertor target.

Thermo-hydraulic instability of boiling natural circulation loop induced by flashing (analytical consideration)

An analytical study is presented on the thermo-hydraulic stability of a boiling natural circulation loop with a chimney at low pressure start-up. The effect of flashing induced by the pressure drop in the channel and the chimney due to gravity head on the instability is considered. A method to analyze linear stability is developed, in which a drift-flux model is used. The analytical result of a stability map agrees very well with the experimental one obtained in a previous report. Instability does not occur when the heater power is too low to generate voids in the chimney and only natural circulation of single phase can be induced. Instability tends to occur when boiling occurs only near the chimney exit due to flashing. This instability phenomenon has some similarities with density wave oscillation, such as the phase difference of temperature between the boiling region and non-boiling region, and the oscillation period which is near to the time required for fluid to pass through the chimney. However, there are also some differences from density wave oscillation, such as the boiling region is very short, and pressure fluctuation can affect void fraction fluctuation.

Heat transfer through subcooled He I layer from distributed heat source in a pressurized He II channel

The subcooled He I layer, in contact with a large heated surface in a channel filled with the pressurized superfluid He II (He II P), expands the non-boiling region above the Kapitza region up to q n , above which nucleate boiling sets in. As the bath temperature decreases, q n is increased more rapidly than q λ at which the superfluidity is broken at the centre of the heated surface. The value of q n is increased as the channel gap increases, and is independent of the channel orientation as well as q λ . Metastabilization of superconducting coils may be enhanced by taking the non-boiling limit q n into account.

超電導コイルの模擬冷却流路における加圧超流動ヘリウムの熱伝達特性

The characteristics of heat transfer in a simulated channel for a large superconducting coil have been investigated. Subcooled He I layer on the heated surface exposed to the pressurized superfluid helium (He IIp) expands the non-boiling region beyond the Kapitza region up to qn, above which nucleate boiling sets in. As the bath temperature decreases, the orientation-independent qn is increased more rapidly than qλ, at which the superfluidity is broken at the center of the heated surface. A decrease in the temperature of the coolant in contact with the heated surface has been detected in He IIp as well as in the saturated He II. Enhanced stabilization can be established for superconducting coils by taking qn into account; the temperature rise at the hot spot on the superconductor may be restrained below 4.5K without bubbling.

Experiments on heat transfer of smooth and swirl tubes under one-sided heating conditions

To design the divertor plate for the next generation fusion machines which is subjected to high heat loads on its one side by the plasma, it is essential to evaluate performances of heat transfer efficiency. However there is little in the literature for predicting of heat transfer coefficients under such one-sided heating conditions. To establish the heat transfer correlation for water under one-sided heating conditions, the authors have performed heat transfer experiments on smooth circular and swirl tubes in the regions from non-boiling to high subcooled partial nucleate boiling. Based on the experimental results, it is confirmed that the existing heat transfer correlations can be applicable at the non-boiling region. For the subcooled partial nucleate boiling region, they cannot be available so that a new heat transfer correlation has been proposed under one-sided heating conditions.

495. 円環流路におけるディフェニールの強制対流沸騰熱伝達

This paper deals with forced convective boiling heat transfer to diphenyl flowing upwards under atmospheric pressure through two vertical concentric annuli 1m long and of effective diameters measuring either 11.4 or 18.0mm. The inlet temperature of diphenyl was 215°-245°C, the Reynolds number 1.9×104-5.7×104, and the heat flux was less than 4.2×105kcal/mm2·hr.Measurements were made of the turbulent heat transfer in the non-boiling region, the heat flux at incipient boiling, the hysteresis of boiling heat transfer, the relationship between boiling heat transfer and the flow pattern of two-phase flow, the effects of fouling, and the burnout heat flux. Correlations of experimental data and some discussions are presented on these items.