Electric field effects on boiling heat transfer of liquid nitrogen

This paper describes an experimental study into the influence of a d.c. uniform electric field on boiling heat transfer characteristics of liquid nitrogen. The electric field effect on the onset of boiling, nucleate boiling curve, hysteresis phenomenon and critical heat transfer were quantitatively investigated using a mesh-plane electrode system. For this purpose, a novel copper block electrode with temperature measurement and vacuum heat insulation was designed and manufactured. Moreover, a heat transfer model based on this electrode geometry has been developed in order to provide some useful data for the electrode design and assist temperature calculations. Detailed heat transfer processes appearing on the copper surface were observed and monitored using a high-speed camera. The experimental results show that electric fields have an obvious effect on boiling heat transfer of liquid nitrogen.     

Thermal performance of cold storage in thermal battery for air conditioning

This article studies, experimentally and theoretically, the thermal performance of cold storage in thermal battery for air conditioning. Thermal battery utilizes the superior heat transfer characteristics of heat pipe and eliminates drawbacks found in the conventional thermal storage tank. Experimental investigations are first conducted to study the cold storage thermal performance in two experimental systems: the ratio of distance between heat pipes to outer diameter of heat pipe W/D=6 and 2. Different heat transfer mechanisms including nucleate boiling, geyser boiling and natural convection are identified in different experimental systems with various liquid fills. A theoretical model to determine the thermal characteristics of the thermal battery has also been developed. Comparisons of this theory with experimental data show good agreements in the nucleate boiling stage of cold storage process.     

Heat transfer to helium-I during different boiling regimes at high centrifugal accelerations

Heat transfer during nucleate and film boiling of helium and also the boiling crises up to relative accelerations of (1 ÷ 2) × 10³ have been investigated. The heat transfer surface was a flat copper heater. The heat transfer during nucleate boiling proved to be independent of the relative accelerations. For film boiling, it was found that $\text{α ～ η}{}^{\mathrm{<mtext>1</mtext><mtext>3</mtext>}}$. The dependence of the peak nucleate boiling heat flux and the minimum film boiling flux on the relative acceleration is non-monotonic.     

Influence of oil on nucleate pool boiling heat transfer of refrigerant on metal foam covers

Nucleate pool boiling heat transfer characteristics of refrigerant/oil mixture on metal foam covers were experimentally investigated. The refrigerant is R113, and the oil is VG68. The copper foams, having ppi (pores per inch) of 10 and 20, porosity from 90% to 98%, and thickness of 5 mm, are selected in this study. Experimental conditions include a saturation pressure of 101 kPa, oil concentrations from 0 to 5%, and heat fluxes from 0 to 80 kW m⁻². The experimental results indicate that the nucleate pool boiling heat transfer coefficient on copper foam covers is larger than that on flat heated surface by a maximum of 160% under the present experimental conditions; the presence of oil deteriorates the nucleate pool boiling heat transfer on copper foam covers by a maximum of 15% under the present experimental conditions, and the deterioration of oil on nucleate pool boiling heat transfer on copper foam covers is lower than that on a flat heated surface. A correlation for predicting the nucleate pool boiling heat transfer coefficient of refrigerant/oil mixture on copper foam cover is developed, and it agrees with 95% of the experimental data within a deviation of ±20%.     

Nucleate boiling of low-concentration alcohol aqueous solutions for the development of thermal management systems in space

To develop high-performance space thermal management systems using boiling phenomena, the heat transfer characteristics of low concentration alcohol aqueous solutions were investigated on ground. For mixtures of 1-Propanol/Water, 2-Propanol/Water and Water/1-Butanol, i.e. substantially positive mixtures at very low concentration range of alcohol, heat transfer enhancement was observed, while only heat transfer deterioration was reported in most of existing studies for nucleate boiling of mixtures. A concept of coexisting heat transfer enhancement due to Marangoni effect additionally to the heat transfer deterioration due to mass transfer resistance was emphasized. The concept seems to be true for mixture nucleate boiling independent of gravity level.     

Transient nucleate boiling of liquid nitrogen with a stepwise change of heat flux

Results of experimental and theoretical investigations of unsteady nucleate boiling of liquid nitrogen are presented. A strip of niobium foil placed along the axis of a tube served simultaneously as a heater and as a resistance thermometer. Noted deviations from correlations for the steady nucleate boiling are explained by a new methematical model, which considers the process of transient nucleate boiling as stochastic.     

Influence of the heat flux in mixture boiling: experiments and correlations

Heat transfer at nucleate pool boiling of the binary and ternary refrigerant mixtures R404A, R407C and R507 at the outside of a horizontal tube with emery ground surface has been investigated in a wide range of pressures and heat fluxes. Together with experimental data of Bednar and Bier for wide boiling ethane/n-butane mixtures, the results of these comparatively narrow boiling mixtures are used to investigate the influence of heat flux q on the heat transfer coefficient as predicted by various correlations for nucleate boiling of mixtures. At comparatively high saturation pressures with experimental -values markedly smaller than the molar average of the pure components, the ,q-relationships predicted differ significantly from the experimental, particularly for wide boiling mixtures.     

Atmospheric and sub-atmospheric boiling of H2O and LiBr/H2O solutionsEbullition à la pression atmosphérique et au-dessous de H2O et de solutions de LiBrH2O

Measurements are presented for the nucleate pool boiling of LiBrH₂O solutions at concentrations typically used in LiBrH₂O air conditioning systems. A commercial grade copper tube was used for all tests as being typical of the testing surface likely to be used in a commercial plant.The object of this work was to correlate nucleate boiling heat transfer for pure water and aqueous solutions using Re, Pr number expressions and thermophysical properties of the boiling liquid and vapour at a clean surface. Such an equation is proposed and tested.     

Flow boiling heat transfer to carbon dioxide: general prediction method

An updated version of the Kattan–Thome–Favrat flow pattern based, flow boiling heat transfer model for horizontal tubes has been developed specifically for CO₂. Because CO₂ has a low critical temperature and hence high evaporating pressures compared to our previous database, it was found necessary to first correct the nucleate pool boiling correlation to better describe CO₂ at high reduced pressures and secondly to include a boiling suppression factor on the nucleate boiling heat transfer coefficient to capture the trends in the flow boiling data. The new method predicts 73% of the CO₂ database (404 data points) to within ±20% and 86% to within ±30% over the vapor quality range of 2–91%. The database covers five tube diameters from 0.79 to 10.06 mm, mass velocities from 85 to 1440 kg m⁻² s⁻¹, heat fluxes from 5 to 36 kW m⁻², saturation temperatures from −25 °C to +25 °C and saturation pressures from 1.7 to 6.4 MPa (reduced pressures up to 0.87).     

Temperature domains on a wire heater

It is demonstrated that nucleate and film boiling in three forms may coexist in the process of boiling on a wire heated by electric current at a preassigned mean temperature of the wire, these forms being a domain (wire segment) of film boiling on a conductor with “cold ends,” a domain of nucleate boiling on a conductor with “hot ends,” and a domain of equilibrium between nucleate and film boiling, at which one conductor end is maintained cold and the other end is maintained hot. The conditions of transition to boiling with domains are analyzed. A calculation technique is suggested, and all three domain branches are constructed. The stability of domain boiling to infinitely small deviations of the wire temperature is investigated. Comparison is made with the available experimental data.     

Heat transfer in nucleate boiling of different low boiling substances

Heat transfer coefficients for nucleate boiling of methane, ethane, ethylene, argon and carbon dioxide were determined using an apparatus for the precise investigation of pool boiling heat transfer in the low temperature range. The apparatus used a horizontal cylinder as the heating element. The influence of the thermophysical properties of the boiling liquid was established by comparing the absolute values of the heat transfer coefficients in a normalized boiling state, i.e. a saturation pressure equal to 10% of the critical pressure and a heat flux density equal to 2 × 10⁴ W m⁻². By including the results for a number of higher boiling liquids, which were investigated previously under similar experimental conditions, and using literature data for three very low boiling liquids, an empirical correlation is established which allows an approximate prediction of the absolute value of the heat transfer coefficient at nucleate boiling for substances of different molecular structure.     

Heat transfer in liquid hydrogen

With boiling of hydrogen under pressure one observes a shift of the q-ΔT curves to higher values of ΔT, caused by a layer of solidified gases, the thickness of which increases continuously. With this process one observes ‘hysteresis’.The departure diameter D and the frequency f of bubbles were observed with discrete nucleate boiling under pressure. The results can be described by the equation f D² = K; K diminishes with rising pressure and with increasing roughness of the surface.     

Modeling of the solid–liquid bubble interface in partial nucleate boiling

Several models of heat transfer in partial nucleate boiling are identified in order to determine the relationship between the dominant physical parameters. The correlations are different for different models, so the main goal of this analysis is to determine the validity of each model and to identify the most dominant physical phenomenon in the nucleate boiling heat transfer. This is done by comparing the results of different models with a vast range of reliable experimental data. The comparison shows that the Sakashita and Kumada model gives the best results in the nucleate boiling heat transfer. It is also shown that the most dominating phenomenon in isolated partial bubbles zones is the transient conduction taking place mainly under the bubbles. This is in contradiction with a majority of the models that consider convection as the most important mode in the nucleate boiling heat transfer. The selected model can also be extrapolated and used in the case of fully developed bubbles zones.     

Partial Nucleate Pool Boiling at Low Heat Flux: Preliminary Ground Test for SOBER-SJ10

Focusing on partial nucleate pool boiling at low heat flux, SOBER-SJ10, one of 27 experiments of the program SJ-10, has been proposed to study local convection and heat transfer around an isolated growing vapor bubble during nucleate pool boiling on a well characterized flat surface in microgravity. An integrated micro heater has been developed. By using a local pulse overheating method in the experimental mode of single bubble boiling, a bubble nucleus can be excited with accurate spatial and temporal positioning on the top-side of a quartz glass substrate with a thickness of 2 mm and an effective heating area of 4.5 mm in diameter, and then grows under an approximate constant heat input provided by the main heater on the back-side of the substrate. Ten thin film micro-RTDs are used for local temperature measurements on the heating surface underneath the growing bubble. Normal pool boiling experiments can also be carried out with step-by-step increase of heating voltage. A series of ground test of the flight module of SOBER-SJ10 have been conducted. Good agreement of the measured data of single phase natural convection with the common-used empirical correlation warrants reasonable confidence in the data. It is found that the values of the incipience superheat of pool boiling at different subcooling are consistent with each others, verifying that the influence of subcooling on boiling incipience can be neglected. Pool boiling curves are also obtained, which shows great influence of subcooling on heat transfer of partial nucleate pool boiling, particularly in lower heat flux.     

Investigations on the heat transfer of boiling binary refrigerant mixturesEtudes sur la transmission de chaleur des mélanges de fluides frigorigènes binaires en ébullition

Heat transfer during nucleate pool boiling was experimentally determined for the mixtures R-12/R-113, R-22/R-12, R-13/R-12, R-13/R-22 and R-23/R-13. For purposes of comparison, the respective five pure refrigerants were also investigated. Dependent upon the mixture, the measurements were made at boiling pressures of p = 0.1 to 2 MPa within the temperature region of t = 198 to 333 (?75° + 60°C) and at heat fluxes of Q = 4 × 10³ to 10⁵ W m^?2. A horizontal, electronically heated copper plate with A = 3 cm² was used. The following quantities were measured: pressure; temperature difference between the heating surface and the boiling liquid; composition and temperature in the liquid and vapour phases; and heat flow rate. The mean error of the heat transfer coefficients found was ± 5%.The results clearly show that the heat transfer for an evaporating mixture deteriorates as compared to the pure components. Essential parameters influencing this reduction are pressure, difference between vapour and liquid composition and heat flux. The fundamental relations and characteristic differences between the individual mixtures are illustrated by figures. The heat transfer coefficients measured can be represented within the whole region studied by a modified relation according to Körner.Observation of the process of evaporation has shown that by agitation (increase of convection) the heat transfer in mixtures can be improved. Additional experiments with evaporation during fluid flow in a pipe are presently in progress.     

Boiling crisis on surfaces with porous coating

Experimental values on nucleate and film boiling of carbon dioxide on polished and porous cylindrical surfaces under conditions of saturation at reduced pressures, P/P_crit=0.93 and 0.97 are discussed. A considerable increase of the maximal, minimal specific heat fluxes and maximal heat transfer coefficient was observed on a horizontal tube with thin porous coating.     

Results of experimental investigation of heat transfer with emulsions with low-boiling disperse phase

The results are given of a theoretical investigation of heat transfer from thin wires to emulsions in which the disperse phase is low-boiling compared to the dispersion medium. A special feature of heat transfer to such emulsions is observed in the mode of nucleate boiling, when only the droplets of the disperse phase of emulsion are boiling. The transition from the convective mode of heat transfer to nucleate boiling requires high values of temperature gradient ΔT = T _W ? T ₀ at which the temperature of the heat-transfer surface T _W is higher than the temperature T _S of saturated vapors of the disperse phase by 100 °C and more. Here, T ₀ is the temperature of the heat-transfer agent away from the heat-transfer surface. Note that, for pure liquids and solutions, the value of delay of the beginning of boiling ΔT _i = T _W ? T _S as a rule does not exceed 1–5 °C. Another feature of heat transfer to the emulsions being treated is a wide temperature range of the mode of nucleate boiling (over 100 °C); the upper limit of this temperature range is defined by the temperature of transition to film boiling of the dispersion medium of the emulsion being investigated that has already boiled by this instant of time. The value of delay of the beginning of boiling is affected significantly by additions of surfactants and adsorbents.     

Characteristic curve of helium pool boiling

The results of an experimental investigation on helium pool boiling over a wide range of temperature differences covering nucleate, transition, and film regions are presented. Heating surfaces under investigation were made of different materials and were processed with various degrees of roughness. Helium boiling curve on stainless steel has an anomalous character: heat flux in the transition region increases when temperature difference rises and there is no nucleate boiling zone on the surface with the lowest degree of roughness. A comparative analysis of the results presented with available data is also carried out.     

Investigation of chemical nucleate boiling

The effect of the solution viscosity and particle size on chemical boiling kinetics is established. A quantitative estimate is obtained for chemical nucleate boiling under conditions of free convection.Translated from Inzhenerno-Fizicheskii Zhurnal, Vol. 52, No. 2, pp. 205–209, February, 1987.     

竖直狭缝通道中液氮沸腾强化换热的实验研究

实验表明，狭缝间隙对液氮自然对流核态沸腾换热有明显的影响，在低热流密度下，间隙小的狭缝沸腾换热比间隙大的狭缝明显增强，当狭缝间隙小于实验压力下气泡的脱离直径时，对于同样的热流密度，传热温差减小一个数量级以上，沸腾换热系数提高十几倍到二十倍以上，当热流密度增加一定程度（＞4W/cm^2)时，间隙小的狭缝沸腾换热比间隙大的狭缝有所减弱。