Pressure dependence of bubble departure diameter for water

Bubble departure diameters have been correlated for water over a wide range of pressures. The averaged deviation of the experimental data from the proposed correlation is approximately ±33% and is typical of most boiling correlations. Considering the variety of the natural surfaces used to obtain experimental data gathered from different sources, it may be noted that the correlation allows a fairly good representation of the available experimental water data.     

Effect of surface roughness & polymeric additive on nucleate pool boiling at subatmospheric pressures

This investigation pertains to boiling heat transfer from a submerged flat surface at subatmospheric and atmospheric pressures in the presence of hydroxy ethyl cellulose (HEC) as a polymeric additive in small doses. Boiling was carried out in presence of the additive on smooth and rough aluminium surfaces having effective cavity size within the range as predicted by Hsu model and the pressure was kept in the range of 8 – 100 KN/sq.m (abs). Effects of surface roughness, saturation pressure and polymer concentration on boiling heat transfer were studied and the results were compared with Rohsenow's correlation.     

Analysis of pool boiling heat transfer: effect of bubbles sliding on the heating surface

Pool boiling on surfaces where sliding bubble mechanism plays an important role has been studied. The heat transfer phenomenon for such cases has been analysed. The model considers different mechanisms such as latent heat transfer due to microlayer evaporation, transient conduction due to thermal boundary layer reformation, natural convection and heat transfer due to the sliding bubbles. Both microlayer evaporation and transient conduction take place during the sliding of bubbles, which occurs in geometries such as inclined surfaces and horizontal tubes. The model has been validated against experimental results from literature for water, refrigerant R134a and propane. The model was found to agree well for these fluids over a wide range of pressures. The model shows the importance of the contributions of the different mechanisms for different fluids, wall superheats and pressures.     

Experimental Research on Performance of Supersonic Steam-Driven Jet Injector and Pressure of Supersonic Steam Jet in Water

An experimental investigation was carried out to find the lifting-pressure performance of a steam-driven jet injector for various configuration parameters. The pressure profiles along the mixing chamber were measured with constant inlet steam and water parameters; the results indicated that the injector had a self-adaptive characteristic and the outlet water mass flow rate had an invariable characteristic. The lifting-pressure coefficient was affected significantly by the area ratio of the secondary nozzle and area ratio of the mixing chamber, and the influences were discussed. Moreover, the pressure distribution characteristic of a supersonic steam jet in water was studied under different steam pressures and water temperatures. The results indicated that the pressure distributions were affected by flow and condensation characteristics that were described by steam pressure and water temperature; the maximum axial pressure ratio was also discussed. Predictions of the distance of the maximum axial pressure by an empirical correlation showed good agreement with the experimental results.     

Mineral scale formation and mitigation on metals and a polymeric heat exchanger surface

An experimental set-up was built to study heat transfer fouling of different pipe materials used in heat exchangers. Fouling mitigation investigations using wood pulp fibres in suspension in the fouling liquid were also performed. The new set-up allows progressive visual observation of fouling with time together with a recorded history under the same solution conditions. On completion, the tube under investigation could be removed to obtain quantitative data on the progressive build up of the deposit as well as the composition of the deposit.The experimental technique involved a pipe test specimen being centrally located in a cylindrical tank concentric with a vertical agitator to give constant and uniform flow conditions near the tube surface. The investigation of calcium sulphate deposition on four different metal surfaces (copper, aluminium, brass and stainless steel SS 316 respectively) and a polycarbonate surface reveals that the fouling increases with time but at a decreasing rate. The deposition on a metal surface can be seen to increase with increasing thermal conductivity and decreasing total surface energy over the range of experiments. Low surface energy material such as polycarbonate causes less attraction to the floating crystals and receives less deposition in comparison to the SS surface.Bleached Kraft softwood fibres at various concentrations were added to the solution to examine their effects on fouling. The results indicate that fouling is reduced as fibre concentration increases. It was also found that the fouling on stainless steel, brass and copper surfaces were all retarded in presence of fibre in the solution.     

Correlation for Heat Transfer in Nucleate Boiling on Horizontal Cylindrical Surface

This experimental investigation deals with nucleate boiling studies on horizontal cylindrical heating elements made out of copper in the medium of Forane around atmospheric conditions. The data could be successfully correlated with the system of criteria employed by the authors in their earlier study of nucleate boiling process on cylindrical heating elements. Inclusion of the data from the present experimental study on Forane and that of other investigators yielded a comprehensive correlation with an average deviation of 20% and standard deviation of 25% over a wide range of system pressures.     

Enhanced Boiling Heat Transfer of the Compact Staggered Tube Bundles under Sub-Atmospheric Pressures

An experimental investigation was carried out on the boiling heat transfer enhancement of water on plain tubes in compact staggered tube-bundle evaporators under atmospheric and sub-atmospheric pressures. The experiment investigated the effects of the tube spacing and positioning and the test pressure on the boiling heat transfer characteristics in restricted spaces of compact tube bundles. The experimental results indicated that for compact tube bundles, the effect of the tube spacing is very significant on the boiling heat transfer. The boiling heat transfer has a maximum enhancement when the tube spacing is so selected as to take an optimum value. The enhanced heat transfer efficiency for the compact bundles would gradually decrease as the test pressure was reduced.     

The boundary layer interaction with shock wave and expansion fan

IntrotcttonThe hypersonic bounce layer develOPment and itsstrUctUre under the influence of distUrbing factorsPresent one of the most complicated problems of fluidmechanics. Such captors affechng the boundal layer arethe adverse andlor favoUrable Pressure gradient,roughness, gas mjechon, cooling, etc. A speCific case ofthis Problem is the study of the flow with successiveimpact of pressure gradients on the bounce layer.Though these factors are intensely stUdied, the queshonof their joint e…     

Measurement of contact angles under condensation conditions. The prediction of dropwise-filmwise transition

Values of contact angles for the systems benzene, carbon tetrachloride, methanol and aniline on P.T.F.E. measured under condensation conditions are presented. The results confirm the role of surface properties in controlling the mechanism of condensation on solid surfaces and with the exception of methanol the contact angles increased as predicted with decreasing pressure. The results indicate that under practical working pressures filmwise condensation of benzene and carbon tetrachloride will always take place on P.T.F.E. surfaces. The pressure at which the contact angle attained the critical value for transition from filmwise to dropwise condensation agreed with previous experimental observations on this system.     

CFD simulation on the performance of twin thermoacoustic prime mover for various resonator lengths and operating pressures

The main objective of the present investigation is to analyze the effect of resonator length and different working pressures on the performance of a twin thermoacoustic prime mover, which was measured in terms of frequency and amplitude of the oscillations using the computational fluid dynamics tool FLUENT 6.3. The simulation was carried out for different resonator lengths such as 0.5, 0.6, 0.8, 1.1, and 1.4 m using nitrogen as a working fluid for various operating pressures such as 1, 2, 3, 4, and 5 bar. It was observed from the results that the pressure amplitude increases with an increase in resonator length and that the output frequency decreases with an increase in resonator length for a fixed operating pressure. For a constant resonator length, the increase in operating pressure has little influence on output frequency, and the pressure amplitude also increases. The results obtained were found to be in accordance with experimental works published by Hariharan and colleagues. 1     

Experimental Investigation on Critical Heat Flux for Water Flowing in a Vertical Uniformly Heated Rifled Tube under Near-critical Pressures

This study experimentally investigated the critical heat flux(CHF) of departure from nucleate boiling(DNB) of water flowing under near-critical pressures in a 2 m-long vertical upward rifled tube with the size of Φ35 × 5.67 mm. Operating conditions included pressures of 18–21 MPa, mass fluxes of 475–1000 kg·m~(-2)·s~(-1), inlet subcooling temperatures of 3–5°C, and wall heat fluxes of 40–960 kW·m~(-2). Tube wall temperature distribution and heat transfer performance in different test conditions were obtained. The effects of the operating parameters on CHF were analyzed. Four heat transfer coefficient correlations were evaluated against our experimental data for further investigation of the two-phase heat transfer characteristics. A heat transfer correlation based on Martinelli number utilized in two-phase region and two empirical correlations used to predict the CHF in rifled tube at near-critical pressures were proposed. Meanwhile, experimental CHF data in rifled tube were compared with six widely used correlations and a CHF look-up table. The CHF enhancement effect in rifled tube is obvious as compared with the CHF data in smooth tube. Results show that DNB occurs at low vapor quality and subcooled region in the rifled tube at near-critical pressures. The increase in pressure leads to the early occurrence of DNB and the decrease in CHF, whereas the increase in mass flux delays the occurrence of DNB and results in the increase in CHF. DNB presents a tendency to move toward the inlet of the rifled tube. At individual operating conditions, DNB and dryout coexist at different sections of the rifled tube.     

Flammability profiles associated with high-pressure hydrogen jets released in close proximity to surfaces

This paper describes experimental and numerical modelling results from an investigation into the flammability profiles associated with high pressure hydrogen jets released in close proximity to surfaces. This work was performed under a Transnational Access Agreement activity funded by the European Research Infrastructure project, H2FC.The experimental programme involved ignited and unignited releases of hydrogen at pressures of 150 and 425 barg through nozzles of 1.06 and 0.64 mm respectively. The proximity of the release to a ceiling or the ground was varied and the results compared with an equivalent free-jet test. During the unignited experiments concentration profiles were measured using hydrogen sensors. During the ignited releases thermal radiation was measured using radiometers and an infra-red camera. The results show that the flammable volume and flame length increase when the release is in close proximity to a surface. The increases are quantified and the safety implications discussed.Selected experiments were modelled using the CFD model FLACS for validation purposes and a comparison of the results is also included in this paper. Similarly to experiments, the CFD results show an increase in flammable volume when the release is close to a surface. The unstable atmospheric conditions during the experiments are shown to have a significant impact on the results.     

Heat transfer by mixed convection in a vertical flow undergoing transition

Heat transfer, for aiding mixed convection from vertical, uniform flux surfaces and for small forced convection effects, is considered here. Simple relations have been proposed to correlate the new experimental data which were obtained in a flow undergoing transition from a laminar regime toward turbulence. Experiments were performed in air at pressures ranging from 4.4 to about 8 bar. The correlation based on experimental data for laminar flow for Pr = 0.7 has been extended to other Prandtl numbers through numerical integration of the transport equations. It is shown that, for both laminar and turbulent mixed convection, the Nusselt number may be successfully correlated, employing suitable combinations of the corresponding heat transfer correlations for forced and for natural convection. The parameter characterizing the mixed convection effect was found to be different in laminar and turbulent flow. However, in each of these regions, the relevant parameter is proportional to the ratio of the applicable characteristic forced and natural convection velocity scales.     

The effect of pressure and surface material on the leidenfrost point of discrete drops of water

The maximum evaporation time and Leidenfrost point for discrete drops of water deposited on smooth surfaces of stainless steel, brass and Monel, at pressures ranging to 75 lb/in2 are obtained and compared. The results suggest that, contrary to expectation, thermal diffusivity of the hot surface is not the controlling factor. The evaporation time-surface temperature correlation due to Baumeister et al. is substantially confirmed.     

Enhancement of pool boiling critical heat flux in dielectric liquids by microporous coatings

An experimental investigation into the effects of pressure and subcooling on the pool boiling critical heat flux from a bare silicon chip-like heater and from a silicon heater coated with microporous layers, is reported. The dual inline heater package was immersed in FC-72, a dielectric fluid, and the experiments were performed in the horizontal orientation, with subcooling varying between 0 K and 72 K, and the pressure between 101.3 kPa and 303.9 kPa. The maximum CHF values on the diamond-base microporous-coated silicon heater were found to reach 47 W/cm², at 3 atm and nearly 50 K of subcooling, and to provide an average enhancement of approximately 60% over the values attained with un-treated silicon surfaces. An available CHF correlation, with a reported standard deviation of 12.5% for un-treated surfaces over a large range of pressures, subcoolings, and surface conditions, was shown to predict the pressure and subcooling effects on CHF from the surface-enhanced chip with a standard deviation of 12%.     

Post-dryout heat transfer to Freon in a vertical tube at high subcritical pressures

Studies of the post-dryout heat transfer were made based on experimental data of the heat transfer to Freon 22 flowing upward in a vertical, round tube at nigh subcritical pressures (reduced pressures of 0.68–0.92). A conventional theoretical model failed in part to reproduce the measured wall temperature. A nondimensional parameter Kn was introduced to a model for the post-dryout regime to take account of the thermodynamic nonequilibrium between the vapor and the liquid droplets. A correlation of Kn was developed and a method using this correlation was successful in predicting the wall temperature.     

Comparison of CHF Enhancement on Microstructured Surfaces With a Predictive Model

An experimental investigation on the role of surface structure in pool boiling critical heat flux (CHF) enhancement on randomly roughened and structured surfaces is considered. In the first set of experiments, CHF was measured on a horizontal circular brass surface with root mean square (RMS) surface roughness varying from 0.15 to 5 μm using pentane, hexane, and FC-72 as working fluids at system pressures varying from 150 to 450 kPa. CHF is observed to increase with increasing surface roughness, although the enhancement diminishes with increasing pressure. A maximum enhancement factor of 1.15 is observed for the surface with 5 μm RMS roughness. CHF enhancement was measured for a second set of experiments with FC-72 and hexane on new reentrant interconnected microstructured hoodoo surfaces, and hoodoo sizes range from 10 to 80 μm. The measured CHF enhancement factor on the hoodoo surfaces varies from 1.05 to 1.67. The enhancement generally increases with decreasing hoodoo size, and the maximum enhancement factor (1.67) is for hexane with 10-μm hoodoos. The maximum enhancement factor for FC-72 is 1.48, also with 10-μm hoodoos. It has been demonstrated that the surfaces which show good CHF enhancement exhibit excellent wicking properties. A recent lift-off CHF model for pool boiling is modified to account for the wicking action observed for structured surfaces. Using the measured wicking rate for various surface/fluid combinations, the lift-off model gives reasonably good prediction of the measured CHF enhancement.     

强化传热管束狭窄空间内R_11的沸腾换热特性

对紧凑型滚压面传热管管束狭窄空间内R－11的沸腾强化换热进行了实验研究,确认了由紧凑型滚压强化管束组成的满液式蒸发换热器具有良好的换热性能。其原理是利用强化传热管管束狭窄空间提前从自然对流换热转换为旺盛核沸腾换热,实验结果确认了管束形成的狭窄空间和强化传热面两种强化技术对沸腾换热的强化效果不能简单叠加。     

On the verification of the effect of water depth on the performance of basin type solar stills

Many experimental and numerical studies have been done on different configurations of solar stills to optimize the design by examining the effect of climatic, operational and design parameters on its performance. The majority of the investigators presented their results in scatter diagrams rather than correlations. One of the most important of the operational parameters that has received a considerable attention in the literature is the brine depth. A good number of the investigations on the effect of brine depth are cited in this study. For each of these studies, a correlation was developed from the data reported by each study. A concluding correlation from all brine depth data was developed. The correlation showed a decreasing trend in the productivity with the increase in the brine depth. An experimental study was subsequently conducted to verify this trend by an experimental investigation on a solar still that was constructed and tested with five different brine depths, namely 1, 4, 6, 8 and 10 cm. The present study validated the decreasing trend in productivity with the increase of brine depth and showed that the still productivity could be influenced by the brine depth by up to 48%.     

管道截面突缩对爆轰波起爆特性的影响

为研究管道截面突缩对爆轰波起爆特性的影响,在突缩比为5:3的截面突缩管道及直管内对不同初始压力下甲烷氧气预混气体的起爆特性进行了实验研究,利用离子探针获得管道内火焰传播速度,并通过二维数值模拟探究了3种不同突缩比的截面突缩管道内火焰及压力的传播特性.实验结果表明,截面突缩管道内爆轰波起爆距离随着初始压力的降低而逐渐增加...