Electrohydrodynamic (EHD) enhancement of boiling heat transfer of R 113+WT4% ethanol

Nucleate boiling heat transfer for refrigerants, R113, and R113+ wt4% ethanol mixture, an azeotropic mixture under electric field was investigated experimentally in a single-tube shell/ tube heat exchanger. A special electrode configuration which provides a more uniform electric field that produces more higher voltage limit against the dielectric breakdown was used in this study. Experimental study has revealed that the electrical charge relaxation time is an important parameter for the boiling heat transfer enhancement under electric field. Up to 1210% enhancement of boiling heat transfer was obtained for R113+wt4% ethanol mixture which has the electrical charge relaxation time of 0.0053 sec whereas only 280% enhancement obtained for R113 which has relaxation time of 0.97 sec. With artificially machined boiling surface, more enhancement in the heat transfer coefficient in the azeotropic mixture was obtained.     

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.     

Investigation of boiling heat transfer characteristics of two-phase closed thermosyphons with various internal grooves

The boiling heat transfer characteristics of two-phase closed thermosyphons with internal grooves are studied experimentally and a simple mathematical model is developed to predict the performance of such thermosyphons. The study focuses on the boiling heat transfer characteristics of a two-phase closed thermosyphons with copper tubes having 50, 60, 70, 80, 90 internal grooves. A two-phase closed thermosyphon with plain copper tube having the same inner and outer diameter as those of grooved tube is also tested for comparison. Methanol is used as working fluid. The effects of the number of grooves, the operating temperature, the heat flux are investigated experimentally. From these experimental results, a simple mathematical model is developed. In the present model, boiling of liquid pool in the evaporator is considered for the heat transfer mechanism of the thermosyphon. And also the effects of the number of grooves, the operating temperature, the heat flux are brought into consideration. A good agreement between the boiling heat transfer coefficient of the thermosyphon estimated from experimental results and the predictions from the present mathematical model is obtained. The experimental results show that the number of grooves and the amount of the working fluid are very important factors for the operation of thermosyphons. The two-phase closed thermosyphon with copper tubes having 60 internal grooves shows the best boiling heat transfer performance.     

Local pool boiling coefficients on horizontal tubes

Local pool boiling on the outside and inside surfaces of a 51 mm diameter tube in horizontal direction has been studied experimentally in saturated water at atmospheric pressure Much variation in local heat transfer coefficients was observed along the tube periphery On the outside surface the maximum and the minimum are observed at θ=45° and 180°, respectively However, on the inside surface only the minimum was observed at θ=0° Major mechanisms on the outside surface are liquid agitation and bubble coalescence while those on the inside surface are micro layer evaporation and liquid agitation As the heat flux increases liquid agitation gets effective both on outside and inside surfaces The local coefficients measured at θ=90° can be recommended as the representative values of both outside and inside surfaces     

Performance of heat transfer and pressure drop in a spirally indented tube

In an effort to develop a heat transfer enhancement technique for low temperature applications such as utilization of LNG cold energy, an experiment was carried out to evaluate the heat transfer and the pressure drop performance for a spirally indented tube using ethylene-glycol and water solutions and pure water under horizontal single-phase conditions. The test tube diameter was 14.86 mm and the tube length was 5.38 m. Heat transfer coefficients and friction factors for both inner and outer surfaces of the test tube were calculated from measurements of temperatures, flowrates and pressure drops. Correlations of heat transfer coefficients in the spirally indented tube, which were applicable for laminar and turbulent regimes were proposed for inner, and outer surfaces. The correlations showed that heat transfer coefficients for the spirally indented tube were much higher than those for smooth tubes, increased by more than 8 times depending upon the Reynolds number. The correlations were compared with other correlations for various types of surface roughness. The effect of the Prandtl number on the heat transfer characteristics was discussed. The critical Reynolds number from the laminar flow to the turbulent flow inside the spirally indented tube was found to be around Re=1,000.     

Numerical simulation of fully developed flow and heat transfer characteristics in a curved tube with pulsating pressure gradient

Characteristics of fluid flow and convective heat transfer of a pulsating flow in a curved tube have been investigated numerically. The tube wall is assumed to be maintained at a uniform temperature peripherally in a fully developed pulsating flow region. The temperature and flow distributions over a cross-section of a curved tube with the associated velocity field need to be studied in detail. This problem is of particular interest in the design of Stirling engine heat exchangers and in understanding the blood flow in the aorta. The time-dependent, elliptic governing equations are solved, employing finite volume technique. The periodic steady state results are obtained for various governing dimensionless parameters, such as Womersley number, pulsation amplitude ration, curvature ratio and Reynolds number. The numerical results indicate that the phase difference between the pressure gradient and averaged axial velocity increases gradually up to π/2 as Womersley number increases. However, this phase difference is almost independent of the amplitude ratio of pulsation. It is also found that the secondary flow patterns are strongly affected by the curvature ratio and Reynolds number. These, in turn, give a strong influence on the convective heat transfer from the pipe wall to the pulsating flow. The results obtained lead to a better understanding of the underlying physical process and also provide input that may be used to design the relevant system. The numerical approach is discussed in detail, and the aspects that must be included for an accurate simulation are discussed.     

A suggestion to enhance pool boiling heat transfer on a vertical tube surface

To find a way of improving pool boiling heat transfer on a vertical tube surface, a revised annulus has been investigated experimentally. The annulus with closed bottoms has a shorter outer tube than the inside heated tube. For the study, three tube diameters (16.5, 19.1, and 25.4mm) and water at atmospheric pressure were used. The annular gap covers from 3.2 to 19.3mm in size and is generated by several glass tubes, which are fabricated around the heated tube. To clarify effects of the revised annulus on heat transfer, experimental results of the annulus were compared to the data of unrestricted tubes. The heat transfer coefficients for the revised annulus increased remarkably in comparison to the unrestricted tube. This paper was recommended for publication in revised form by Associate Editor Jae Young Lee Myeong-Gie Kang received his B.S. degree in Precision Mechanical Engineering from Pusan National University, Korea, in 1986. He then received his M.S. and Ph.D. degrees from KAIST in 1988 and 1996, respectively. Dr. Kang is currently a Professor at the Department of Mechanical Engineering Education at Andong National University in Andong, Korea. He has served as an engineer and researcher in KEPCO for 7 years. His research interests include pool boiling heat transfer, flow induced vibration, and nuclear thermo-hydraulics.     

The study on pressure oscillation and heat transfer characteristics of oscillating capillary tube heat pipe

In the present study, the characteristics of pressure oscillation and heat transfer performance in an oscillating capillary tube heat pipe were experimentally investigated with respect to the heat flux, the charging ratio of working fluid, and the inclination angle to the horizontal orientation. The experimental results showed that the frequency of pressure oscillation was between 0.1 Hz and 1.5 Hz at the charging ratio of 40 vol.%. The saturation pressure of working fluid in the oscillating capillary tube heat pipe increased as the heat flux was increased. Also, as the charging ratio of working fluid was increased, the amplitude of pressure oscillation increased. When the pressure waves were symmetric sinusoidal waves at the charging ratios of 40 vol.% and 60 vol.%, the heat transfer performance was improved. At the charging ratios of 20 vol.% and 80 vol.%, the waveforms of pressure oscillation were more complicated, and the heat transfer performance reduced. At the charging ratio of 40 vol.%, the heat transfer performance of the OCHP was at the best when the inclination angle was 90°. the pressure wave was a sinusoidal waveform, the pressure difference was at the least, the oscillation amplitude was at the least, and the frequency of pressure oscillation was the highest.     

Fouling effect of geothermal water scale on heat transfer around a tube in a bank

Seales deposited onto the heating surfaces of heat exchangers seriously reduce the heat transfer performance and also increase the hydrodynamic drag. Accordingly, fouling is an important problem in the design and operation of heat exchangers. Present paper investigates the heat transfer around in-line four circular cylinders on which geothermal water scales are uniformly distributed. The cylinders are settled in tandem with equal distance between neighbouring cylinders and only the test cylinder is heated under the condition of constant heat flux. It is found that the heat transfer of the in-line tube banks greatly varies with the fouling of geothermal water scale, especially its scale height. Further, the local and average Nusselt numbers strongly depend upon the cylinder spacing and the Reynolds number.     

静电分离器中电场和电流体特性试验研究

利用静电分离技术从空气中分离平均直径为2μm的油滴,对小尺度的线—板式静电分离器的电晕放电特性和流型特征进行试验研究,放电电极接高压电源正极。测试在不同的气体流速、油滴浓度条件下分离器的伏安特性曲线。利用高速摄像机对其中的电流体流型特征进行可视化研究,试验测定进口流速为0.2 m/s、0.3 m/s、0.4 m/s,施加电压从0～16 kV条件下电流体流型的变化,得到在不同的施加电压和气体流速下的流型分布图。试验表明,进口流速对分离器的电场影响并不明显,而油滴浓度对电场有很大的影响,在相同的施加电压下电流密度随着油滴浓度的增大而降低。电流体流型的变化取决于进口流速和施加电压的相互作用。得到在不同的描述惯性力和电场力关系的量纲一参数雷诺数和电流体数下5种代表的电流体流型。     

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.     

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.     

Two-phase flow heat transfer of propane vaporization in horizontal minichannels

Experiments were performed on the convective boiling heat transfer in horizontal minichannels using propane. The test section was made of stainless steel tubes with inner diameters of 1.5 mm and 3.0 mm and lengths of 1000 mm and 2000 mm, respectively, and it was uniformly heated by applying an electric current directly to the tubes. Local heat transfer coefficients were obtained for a heat flux range of 5–20 kW m⁻², a mass flux range of 50–400 kg m⁻² s⁻¹, saturation temperatures of 10, 5, and 0°C and quality ranges of up to 1.0. The nucleate boiling heat transfer contribution was predominant, particularly at the low quality region. Decreases in the heat transfer coefficient occurred at a lower vapor quality with a rise of heat flux and mass flux, and with a lower saturation temperature and inner tube diameter. Laminar flow appeared in the minichannel flows. A new boiling heat transfer coefficient correlation that is based on the superposition model for propane was developed with 8.27% mean deviation. This paper was recommended for publication in revised form by Associate Editor Jae Young Lee Jong-Taek Oh received his B.S., M.S. and Ph.D. degrees in Refrigeration Engineering from Pukyong National University, Korea. Dr. Oh is currently a Professor at Department of Refrigeration and Air Conditioning Engineering, Chonnam National University at Yeosu, Korea. Dr. Oh’s research interests are in the area of boiling and condensation heat transfer and pressure drop of refrigerants with small tubes, heat pump and transportation refrigeration.     

Analysis of flow and stresses in a tube stretch-reducing hot rolling schedule

Analytical expressions for the roll-pass geometry, velocity, strain, strain rate and stress components are obtained for steady-state tube stretch-reducing hot rolling. From these expressions the reduction in tube diameter and wall thickness, inter-stand tensions, roll load and rolling torque have been determined in relation to the roll gap opening and the inter-stand velocity increase ratio. The results have shown that increasing the interstand velocity increase ratio will increase the inter-stand tension while the roll load remains almost unchanged. The rolling torque increases with the increase of the back inter-stand velocity increase ratio and decreases with the increase of the ratio at the front. There is a limiting value for the inter-stand velocity increase ratio at which the rolling process will tend to become unstable. The analysis has been validated by comparing the theoretical results with on-line power measurements on an industrial tube stretch-reducing hot rolling mill.     

An experimental study on heat transfer characteristics with turbulent swirling flow using uniform heat flux in a cylindrical annuli

An experimental study was performed to investigate heat transfer characteristics of turbulent swirling flow in an axisymmetric annuli. The static pressure, the local flow temperature, and the wall temperature with decaying swirl were measured by using tangential inlet conditions and the friction factor and the local Nusselt number were calculated for Re=30000-70000. The local Nusselt number was compared with that obtained from the Dittus-Boelter equation with swirl and without swirl. The results showed that the swirl enhances the heat transfer at the inlet and the outlet of the test tube.     

管内流体换热性能测试系统

设计了一种测试流体(煤油)管内换热性能的实验系统.该实验系统包括了循环系统、加热系统、冷却系统、增压系统、测量系统(流量测量、压力测量、温度测量)和数据采集系统,各部分做了详细的说明,对其测试原理也作了说明.流体进、出口温度以及预热段流体进口温度都用铠装热偶测量.预热段和实验段都采用电加热方法加热(采用低电压、大电流加热).预热段和实验段都均匀布置20个热电偶测量管外壁温度,电加热管内壁温可通过龙格一库塔方法由外壁温和加热功率求得.该系统也可以用来测量其它流体管内换热性能,还可以用来测量流体的比热;还可以测量流动压降和摩擦阻力系数.     

A study of transient heat transfer from the heater surface to a boiling liquid

A new method for studying transient heat transfer across a boiling liquid-solid interface is described. The method is based on the use of film temperature gauges with a size of 1000 × 10 μm placed on the heated surface. The effectiveness of the method is verified by measurements of temperature pulsations in the boiling liquid nitrogen and that of the transition from the nucleate to film boiling during pulsed heating.     

Laminar forced convective heat transfer to near-critical water in a tube

Numerical modeling is carried out to investigate forced convective heat transfer to nearcritical water in developing laminar flow through a circular tube. Due to large variations of thermo-physical properties such as density, specific heat, viscosity, and thermal conductivity near thermodynamic critical point, heat transfer characteristics show quite different behavior compared with pure forced convection. With flow acceleration along the tube unusual behavior of heat transfer coefficient and friction factor occurs when the fluid enthalpy passes through pseudocritical point of pressure in the tube. There is also a transition behavior from liquid-like phase to gas-like phase in the developing region. Numerical results with constant heat flux boundary conditions are obtained for reduced pressures from 1.09 to 1.99. Graphical results for velocity, temperature, and heat transfer coefficient with Stanton number are presented and analyzed.     

针翅管的强化传热试验研究

对针翅管进行了试验研究、试验介质涉及了气体（空气）、水－油、水－水等,并分析了针翅管的传热性能和阻力压降,得出了相应的流动换热准则关系式,结果表明针翅管结构明显影响流动换热性能。     

Experimental studies on heat transfer in the annuli with corrugated inner tubes

Experimental heat transfer data for single-phase water flow in the annuli with corrugated inner tubes are presented. In the annuli with parallel flow, ten different annular arrangements are considered. For water (low rate in l,700<Re<13,000 regime, data for Nusselt numbers are presented. The results show significant effects of both the pitch to trough height ratio (P/e) and the radius ratio (r ^*). AsP/e becomes closer to 8 in the range below the radius ratio(r ^*) of 0.5. Nusselt numbers increase. However, Nusselt numbers decrease in the range above the radius ratio (r^*) of 0.5 because flow reattachment position becomes farther in the narrower clearance.