Enhancement of pool boiling heat transfer coefficients using carbon nanotubes

In this study, the effect of carbon nanotubes (CNTs) on nucleate boiling heat transfer is investigated. Three refrigerants of R22, R123, R134a, and water were used as working fluids and 1.0 vol.% of CNTs was added to the working fluids to examine the effect of CNTs. Experimental apparatus was composed of a stainless steel vessel and a plain horizontal tube heated by a cartridge heater. All data were obtained at the pool temperature of 7°C for all refrigerants and 100°C for water in the heat flux range of 10–80 kW/m². Test results showed that CNTs increase nucleate boiling heat transfer coefficients for all fluids. Especially, large enhancement was observed at low heat fluxes of less than 30 kW/m². With increasing heat flux, however, the enhancement was suppressed due to vigorous bubble generation. Fouling on the heat transfer surface was not observed during the course of this study. Optimum quantity and type of CNTs and their dispersion should be examined for their commercial application to enhance nucleate boiling heat transfer in many applications.     

Experimental study on CuO nanoparticles in distilled water and its effect on heat transfer on a vertical surface

The pool boiling characteristics of dilute dispersions of CuO nanoparticles in water were studied at atmospheric pressure on a vertical heating surface. Experimental investigation of different weight concentrations of nanoparticles revealed significant enhancement in heat flux and deterioration in pool boiling. Out of many reasons, nanoparticles coating the heater surface was believed to be the reason behind this. Subsequent inspection of the heater surface showed nanoparticles coating the surface, forming a porous layer. To substantiate the nanoparticle deposition and its effect on heat flux, an investigation was performed by measuring the surface roughness of the heater surface before and after the experiment. While SEM images of the heater surface revealed nanoparticle deposition, measurement of surface roughness of the heater surface confirmed it. Formation of the porous layer on the heater surface as revealed by SEM images provided an excellent location for nucleation sites enhancing heat transfer. However, deterioration in nucleate boiling at different weight concentrations indicated some phenomenon is working behind this.     

The effect of surface area on pool boiling heat transfer coefficient and CHF of Al2O3/water nanofluids

In this study, pool-boiling experiments are carried out to find out the influence of nanoparticles on boiling heat transfer coefficient and CHF. Each surface of the heater is visualized with FE-SEM and AFM after the experiments to figure out the effective boiling areas. The CHF increases up to 103% (compared to pure water) as the particle concentration increases until 0.001 vol% while it starts to decrease gradually as the particle concentration increases more than 0.001 vol%. It is found that the increase of CHF is proportional to the effective boiling surface area and the reduction of boiling heat transfer coefficient (BHTC) is mainly attributed to the blocking of the active nucleation cavity and the increase of the heat transfer resistance by nanoparticle deposition on the boiling surface. Finally, this study proposes a novel mechanism for BHTC reduction and CHF enhancement by nanofluids by considering the effective surface area variation.     

Pool boiling heat transfer to zinc oxide-ethylene glycol nano-suspension near the critical heat flux

Experimental study is conducted on the thermal performance of the ZnO nanoparticles dispersed in water-ethylene glycol (water/EG) as the base fluid under the pool boiling condition. Experimental facility provides condition to apply heat fluxes up to the critical heat flux point. Influence of different operating parameters such as heat flux and mass concentration on the boiling heat transfer coefficient and thermal fouling resistance is investigated. Results showed that with an increase in heat flux and mass concentration, the heat transfer coefficient increases; however, at concentration of 0.5 % by weight for ZnO/water/EG nano-fluid. Deposition of particles on the heating surface was a significant disadvantage of the nano-fluid. However, with an increase in mass concentration of nanoparticles, the critical heat flux point is enhanced, which is due to the enhancement in deposition layer of the nanoparticles, resulting in capillary wick and keeping more liquid inside the deposition layer.

     

Effect of surface roughness on pool boiling heat transfer in subcooled water-CuO nanofluid

We investigated the effect of surface roughness on pool boiling heat transfer in subcooled water-CuO nanofluid. Experiment was performed using 0.1% volumetric water-CuO nanofluid and pure water for comparison. The following results were obtained. The heat flux tended to increase as the liquid subcooling increased in the region of low wall superheat. However, the effect of liquid subcooling gradually decreased as the wall superheat increased. The heat flux of pure water and nanofluid was almost similar in the region of low wall superheat. As the wall superheat increased, however, the heat flux of nanofluid decreased compared to that of pure water. This was attributable to the fact that the nanoparticles mixed with pure water reduced the heat flux by deteriorating boiling on the heat transfer surface. The heat flux increased as the surface roughness increased in the pure water, but the effect of surface roughness on heat flux was unclear in the nanofluid. This was attributable to the decreased difference of surface roughness, which was caused by the coating or deposition of nanoparticles on the heat transfer surface during the experiment.     

Enhancement of the critical heat flux by using heat spreader

Direct immersion cooling has been considered as one of the promising methods to cool high power density chips. A fluorocarbon liquid such as FC-72, which is chemically and electrically compatible with microelectronic components, is known to be a proper coolant for direct immersion cooling. However, boiling in this dielectric fluid is characterized by its small value of the critical heat flux. In this experimental study, we tried to enhance the critical heat flux by increasing the nucleate boiling area in the heat spreader (Conductive Immersion Cooling Module). Heat flux of 2 MW/m² was successfully removed at the heat source temperature below 78°C in FC-72. Some modified boiling curves at high heat flux were obtained from these modules. Also, the concept of conduction path length is very important in enhancing the critical heat flux by increasing the heat spreader surface area where nucleate boiling occurs.     

An experimental study of critical heat flux in non-uniformly heated vertical annulus under low flow conditions

An experimental study on critical heat flux (CHF) has been performed in an internally heated vertical annulus with non-uniform heating. The CHF data for the chopped cosine heat flux have been compared with those for uniform heat flux obtained from the previous study of the authors, in order to investigate the effect of axial heat flux distribution on CHF. The local CHF with the parameters such as mass flux and critical quality shows an irregular behavior. However, thetotal critical power with mass flux and theaverage CHF with critical quality are represented by a unique curve without the irregularity. The effect of the heat flux distribution on CHF is large at low pressure conditions but becomes rapidly smaller as the pressure increases. The relationship between the critical quality and the boiling length is represented by a single curve, independent of the axial heat flux distribution. For non-uniform axial heat flux distribution, the prediction results from Doerffer et al.’s and Bowring’s CHF correlations have considerably large errors, compared to the prediction for uniform heat flux distribution. KeyWords : Critical Heat Flux, Heated Vertical Annulus, Low Mass Flux, Wide Range Pressure, Non-uniform Heating, Effect of Axial Heat Flux Distribution, Boiling Length     

A review of the critical heat flux condition in mini-and microchannels

With ever increasing power dissipation in electronic chips that are shrinking in size, cooling demands are becoming more severe. Forced air cooling is reaching its operational limits, and single-phase liquid cooling in microchannels has been able to accommodate the rising heat fluxes. Further increases in computing (chip) power suggest that a switch from single-phase to boiling heat transfer will be needed. A major impediment to using boiling or forced convective vaporization for such a cooling application is the limiting critical heat flux (CHF) condition. In this paper, the CHF condition in microchannels is reviewed. Data from the literature are discussed, and new data for a range of operating and geometric conditions are presented. Influencing factors, parametric trends, phenomenological models, and other aspects of the CHF condition are discussed.     

低压条件下紧凑叉排管束沸腾换热特性试验

针对传统的满液型蒸发换热器,将蒸发器中的水平加热管束按叉排方式紧凑排列形成窄缝空间,在大气压和低压运行条件下,利用窄缝空间沸腾强化换热机理,可以将在低壁温/低热负荷条件下的自然对流换热转化为核态沸腾换热,能有效提高满液式蒸发器的换热性能。和传统的满液型蒸发换热器相比,这种紧凑式蒸发器平均换热系数能提高一倍以上。紧凑蒸发器的管距、管位置,工作压力都对蒸发器的换热性能有显著影响,管距的影响是最大的。不同的压力条件下存在一个对应的最佳管距。在此管距下,蒸发换热器的强化换热性能达到最大。最佳管距对应的管束水力当量直径近似等于池内沸腾时的气泡脱离直径。随着压力减小,最佳管距逐渐增大。同时,紧凑式管束布置引起的窄缝空间内沸腾强化换热强化效果也逐步降低。     

Heat transfer performance of jet impingement flow boiling using Al2O3-water nanofluid

An investigation is performed into the heat transfer performance of jet impingement flow boiling using Al₂O₃-water nanofluids with Al₂O₃ additions of 0, 0.0001, 0.001 and 0.01 vol%, respectively. It is shown that the heat transfer performance of jet impingement flow boiling using Al₂O₃-water nanofluid is poorer than that obtained when using de-ionized (DI) water as the working fluid. Scanning electron microscopy (SEM) and energy-dispersive X-ray spectrometry (EDS) observations reveal that the reduction in the heat transfer performance is due to the formation of a nano-sorption layer on the heated surface, which results in an increase in the thermal resistance. However, it is shown that by applying acoustic vibration to the heated surface, the formation of the nano-sorption layer is prevented; with the result that the heat transfer performance obtained using the Al₂O₃-water nanofluids is better than that obtained using pure DI water.     

Nucleate Boiling Heat Transfer Coefficients of Mixtures Containing Propane, Isobutane and HFC134a

Nucleate pool boiling heat transfer coefficient (HTCs) were measured with one nonazeotropic mixture of propane/isobutane and two azeotropic mixtures of HFC134a/isobutane and propane/HFC 134a. All data were taken at the liquid pool temperature of 7°C on a horizontal plain tube of 19.0 mm outside diameter with heat fluxes of 10kW/m² to 80 kW/m² with an interval of 10 kW/m² in the decreasing order of heat flux. The measurements were made through electrical heating by a cartridge heater. The nonazeotropic mixture of propane/isobutane showed a reduction of HTCs as much as 41% from the ideal values. The azeotropic mixtures of HFC134a/isobutane and propane/HFC 134a showed a reduction of HTCs as much as 44% from the ideal values at compositions other than azeotropic compositions. At azeotropic compositions, however, the HTCs were even higher than the ideal values due to the increase in the vapor pressure. For all mixtures, the reduction in heat transfer was greater with larger gliding temperature difference. Stephan and Körner’s and Jung et al’s correlations predicted the HTCs of mixtures with a mean deviation of 11 %. The largest mean deviation occurred at the azeotropic compositions of HFC 134a/isobutane and propane/HFC 134a.     

Experimental study of heating surface angle effects on single bubble growth

Nucleate pool boiling experiments were performed using pure R11 for various surface angles under constant heat flux conditions during saturated pool boiling. A 1-mm-diameter circular heater with an artificial cavity in the center that was fabricated using a MEMS technique and a high-speed controller were used to maintain the constant heat flux. Bubble growth images were taken at 5000 frames per second using a high-speed CCD camera. The bubble geometry was obtained from the captured bubble images. The effects of the surface angle on the bubble growth behavior were analyzed for the initial and thermal growth regions using dimensional scales. The parameters that affected the bubble growth behavior were the bubble radius, bubble growth rate, sliding velocity, bubble shape, and advancing and receding contact angles. These phenomena require further analysis for various surface angles and the obtained constant heat flux data provide a good foundation for such future work.     

Numerical study on a sliding bubble during nucleate boiling

A numerical method for simulating bubble motion during nucleate boiling is presented. The vapor-liquid interface is captured by a level set method which can easily handle breaking and merging of the interface and can calculate an interfacial curvature more accurately than the VOF method using a step function. The level set method is modified to include the effects of phase change at the interface and contact angle at the wall as well to achieve mass conservation during the whole calculation procedure. Also, a simplified model to predict the heat flux in a thin liquid microlayer is developed. The method is applied for simulation of a sliding bubble on a vertical surface to further understand the physics of partial nucleate boiling. Based on the computed results, the effects of contact angle, wall superheat and phase caange on a sliding bubble are quantified.     

A theoretical model of critical heat flux in flow boiling at low qualities

A new theoretical critical heat flux (CHF) model was developed for the forced convective flow boiling at high pressure, high mass velocity, and low quality. The present model for an intermittent vapor blanket was basically derived from the sublayer dryout theory without including any empirical constant. The vapor blanket velocity was estimated by an axial force balance, and the thickness of vapor blanket was determined by a radial force balance for the Marangoni force and lift force. Based on the comparison of the predicted CHF with the experimental data taken from previous studies, the present CHF model showed satisfactory results with reasonable accuracy.     

An experimental investigation of heat transfer in forced convective boiling of R134a,R123 and R134a/R123 in a horizontal tube

This paper reports an experimental study on flow boiling of pure refrigerants R134a and R123 and their mixtures in a uniformly heated horizontal tube. The flow pattern was observed through tubular sight glasses with an internal diameter of 10 mm located at the inlet and outlet of the test section. Tests were run at a pressure of 0.6 MPa in the heat flux ranges of 5–50 kW/m², vapor quality 0–100 percent and mass velocity of 150–600 kg/m²s. Both in the nucleate boiling-dominant region at low quality and in the two-phase convective evaporation region at higher quality where nucleation is supposed to be fully suppressed, the heat transfer coefficient for the mixture was lower than that for an equivalent pure component with the same physical properties as the mixture. The reduction of the heat transfer coefficient in mixture is explained by such mechanisms as mass transfer resistance and non-linear variation in physical properties etc. In this study, the contribution of convective evaporation, which is obtained for pure refrigerants under the suppression of nucleate boiling, is multiplied by the composition factor by Singal et al. (1984). On the basis of Chen’s superposition model, a new correlation is presented for heat transfer coefficients of mixture.     

Comparative study on heat transfer characteristics of nanofluidic thermosyphon and grooved heat pipe

The present study used TiO₂-nanofluid with different volume ratios as the working fluids of a therrmosyphon and grooved heat pipe and investigated various parameters such as volume concentration of nanoparticles, orientation, heat flux, and cooling media. Further, the present study used nanofluids and dispersed TiO₂-nanoparticles into pure water with each cross-blended concentration of 0.05%, 0.1%, 0.5%, and 1%. The authors observed the best heat transfer performance in the 0.05% concentration with thermosyphon. The present study presents the enhancement of heat transfer performance with TiO₂-nanofluids, and fabricated a heat pipe from a straight stainless steel tube with an outer diameter and length of 10 and 500 mm, respectively. At the optimum condition for the pure refrigerant, the thermosyphon with 0.05% TiO₂-nanoparticle concentration gave 1.40 times higher efficiency than that of pure water.     

微通道中液氮的流动沸腾——换热特性分析

对微通道中液氮流动沸腾换热特性进行试验研究和分析。给出典型的沸腾曲线,分析壁温、干度和换热系数沿微通道管程的变化规律,考察热流密度、质量流量和压力对流动沸腾换热的影响。将126个试验数据点与四个换热关联式比较,并对微通道中流动沸腾换热机理进行分析。结果表明,在多数情况下干度和热流密度对沸腾换热系数的影响较小,换热系数主要决定于质量流量和压力,随两者增加而增加,换热以对流蒸发为主导机理。KLIMENKO关联式预测效果最好,TRAN微通道关联式次之,对常规管道得到广泛使用的CHEN关联式和SHAH关联式都远远高估了试验值。基于两相流压降和换热特性分析,推知微通道中的两相流流型不同于常规管道：在低干度情况下,流型以弥散泡状流为主;而在高干度情况下,流型以由雾状汽芯和不规则液膜组成的环状流为主。     

Electrohydrodynamic(ehd) enhancement of boiling heat transfer with a lo-fin tube

The effect of D. C electric field on nucleate boiling heat transfer for refrigerants of R-11, HCFC-123 and FC-72 was investigated experimentally by using a single lo-fin tube shell-and-tube heat exchanger. The lo-fin tube which brought two times increase in the heat transfer area provided about 150% of boiling heat transfer enhancement compared to that of smooth surface. This experimental study has revealed that the electrical charge relaxation time was an important parameter for the boiling heat transfer enhancement under electric field. Boiling heat transfer enhancement was obtained up to 40% for R-11 which had moderate relaxation time of 1.3s. However remarkable boiling heat transfer enhancement has been obtained up to three fold increase(300%) for HCFC-123 which has the electrical charge relaxation time of 0.89 x 10^-3s. For FC-72 having longer relaxation time than the bubble detachment one, no appreciable effect on the nucleate boiling heat transfer was observed.     

水平加热管上气泡动力特性对纳米颗粒悬浮液过冷沸腾的影响

纯水在水平加热管加热下的过冷池沸腾试验揭示出,在热流密度低于24.4 kW/m2时,加热管底部所需的过热度最低,中部次之,顶部最高;随着热流密度的增加,加热管各位置所需的过热度逐渐趋于一致。纳米颗粒悬浮液的试验发现,低热流密度时,各位置处所需过热度的差异较小,但随着热流密度的增加,加热管底部所需的过热度超过了顶部,这种差异随着颗粒浓度的增加而增加。从气泡动力学角度对产生差异的原因进行了分析,主要的结论是,考虑纳米颗粒/团聚体在加热管顶部表面的沉积/吸附,会降低加热管表面粗糙度,同时使得悬浮液微液底层内出现一个吸引力Fa,从而使得悬浮液的相变温度提高,而由于加热管中、底部分粗糙系数降低所带来的换热降低较加热管顶部要强,使得大部分热流密度下加热管顶部沸腾所需的过热度较中、底部分要高。     

Condensation heat transfer coefficients of flammable refrigerants on various enhanced tubes

In this study, external condensation heat transfer coefficients (HTCs) of six flammable refrigerants of propylene (R1270), propane (R290), isobutane (R600a), butane (R600), dimethylether (RE170), and HFC32 were measured at the vapor temperature of 39°C on a 1023 fpm low fin and Turbo-C tubes. All data were taken under the heat flux of 32- 116 and 42-142 kW/m² for the Iow fin and Turbo-C tubes respectively. Flammable refrigerants’ data obtained on enhanced tubes showed a typical trend that external condensation HTCs decrease with increasing wall subcooling. HFC32 and DME showed up to 30% higher HTCs than those of HCFC22 due to their excellent thermophysical properties. Propylene, propane, isobutane, and butane showed similar or lower HTCs than those of HCFC22. Beatty and Katz’ correlation predicted the HTCs of the flammable refrigerants obtained on a low fin tube within a mean deviation of 7.3%. Turbo-C tube showed the best performance due to its 3 dimensional surface geometry for fast removal of condensate.