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Dry-out CHF correlation for R134a flow boiling in a horizontal helically-coiled tube

  • Authors: Chen, C.-N. , Han, J.-T. , Jen, T.-C. , Shao, L.
  • Date: 2011
  • Subjects: Critical heat flux , Helically coiled tubes , Heat transfer
  • Type: Article
  • Identifier: uj:5286 , http://hdl.handle.net/10210/14955
  • Description: An experimental study was carried out to investigate the R134a dry-out critical heat flux (CHF) characteristics in a horizontal helically-coiled tube. The test section was heated uniformly by DC high-power source, and its geometrical parameters are the outer diameter of 10 mm, inner diameter of 8.4 mm, coil diameter of 300 mm, helical pitch of 75 mm and valid heated length of 1.89 m. The experimental parameters are the outlet pressures of 0.30–0.95 MPa, mass fluxes of 60–500 kg m 2 s 1, inlet qualities of 0.36–0.35 and heat fluxes of 7.0 103–5.0 104 Wm 2. A method based on Agilent BenchLink Data Logger Pro was developed to determine the occurrence of CHF with a total of 68 T-type thermocouples (0.2 mm) set along the tube for accurate temperature measurement. The characteristics of wall temperatures and the parametric effect on dry-out CHF showed that temperature would jump abruptly at the point of CHF, which usually started to form at the front and offside (270 and 90 ) of the outlet crosssection. The CHF values decrease nearly linearly with increasing inlet qualities, while they decrease more acutely with increasing critical qualities, especially under larger mass flux conditions. The mass flux has a positive effect on CHF enhancement, but the pressure has negative one. A new dimensionless correlation was developed to estimate dry-out CHF of R134a flow boiling in horizontal helically-coiled tubes under current experimental conditions and compared to calculated results from Bowring and Shah correlations.
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Thermo-chemical characteristics of R134a flow boiling in helically coiled tubes at low mass flux and low pressure

  • Authors: Chen, Chang-Nian , Han, Ji-Tian , Jen, Tien-Chien , Shao, Li
  • Date: 2011
  • Subjects: Helically coiled tubes , Heat transfer , Low mass flux , Low pressure
  • Type: Article
  • Identifier: uj:5282 , http://hdl.handle.net/10210/14951
  • Description: The characteristics of R134a heat transfer coefficients and wall temperature distribution were investigated under low mass flux and low pressure conditions in a helically coiled tube with heated length of 7070mm, outer diameter of 10mm, inner diameter of 7.6mm, coil diameter of 300mm and helical pitch of 40mm. System pressures, mass fluxes and inlet qualities range from 0.20 to 0.75 MPa, 50 to 260 kg/m2 s and −0.18 to 0.40, respectively. It was found that the wall temperatures in descending segments of coiled tube were higher than those of climbing ones, while the heat transfer coefficients varied inversely. Around the section circumference, the outside temperature was lower than the inside one; this is more apparent at very low mass flux and pressure conditions. The heat transfer coefficient increases with increasing mass flux, vapor quality and heat flux. However, the pressure has an indeterminate effect. New heat transfer coefficient correlations for current conditions were developed comparing with existing correlations.
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Experimental study on critical heat flux characteristics of R134a flow boiling in horizontal helically-coiled tubes

  • Authors: Chen, Chang-Nian , Han, Ji-Tian , Jen, Tien-Chien , Shao, Li , Chen, Wen-wen
  • Date: 2011
  • Subjects: Critical heat flux , Helically coiled tubes , Heat transfer
  • Type: Article
  • Identifier: uj:5284 , http://hdl.handle.net/10210/14953
  • Description: Critical heat flux (CHF) experiments were performed to study the R134a CHF characteristics in horizontal helically-coiled tubes. The stainless steel test sections were heated uniformly, with tube inner diameters of 3.8e11 mm, coil diameters of 135e370 mm, helical pitches of 40e105 mm and heated lengths of 0.85e7.54 m. The experimental conditions are pressures of 0.30e1.10 MPa, mass fluxes of 60e480 kg m 2 s 1, inlet qualities of 0.32e0.36 and heat fluxes of 6.0 103e9.0 104Wm 2. It was found that the wall temperatures jumped abruptly once the CHF occurred. The CHF values decrease with increasing heated lengths, coil diameters and inner diameters, but the DNB (departure from nucleate boiling) CHF seems independent when length-to-diameter L/di> 200. The coil-to-diameter ratios are more important than length-to-diameter ratios for CHF in helically-coiled tubes, while the helical pitches have little effect on CHF. The CHF value increases greatly with increasing mass flux and decreases smoothly with increasing pressure. It decreases nearly linearly with increasing inlet and critical qualities, but it varies more acutely at xcr< 0.5 than higher critical qualities. New correlations for R134a flow boiling CHF in horizontal helically-coiled tubes were developed for applications.
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A heat transfer correlation of flow boiling in micro-finned helically coiled tube

  • Authors: Cui, Wenzhi , Li, Longjian , Xin, Ming-dao , Jen, Tien-Chien , Chen, Qinghua , Liao, Quan
  • Date: 2006
  • Subjects: Convective boiling , Heat transfer , Helically coiled tubes , Nucleate boiling
  • Type: Article
  • Identifier: uj:5275 , http://hdl.handle.net/10210/14944
  • Description: Two main mechanisms, nucleate boiling and convective boiling, are widely accepted for in-tube flow boiling. Since the active nuclei on the heated wall are dominant for nucleate boiling and flow pattern governs the convective boiling, the heat transfer coefficient is strongly influenced by the wall heat flux, mass flux and vapor quality, respectively. In practical industrial applications, for example, the evaporators in refrigeration, forced convective evaporation is the dominant process and high heat transfer efficiency can be obtained under smaller temperature difference between wall and liquid. Therefore, it is of importance to develop a correlation of convective boiling heat transfer with a good accuracy. In this paper, a new kind of micro-finned helically coiled tube was developed and the flow boiling heat transfer characteristics were experimentally studied with R134a. Based on the analysis of the mechanisms of flow boiling, heat transfer correlations of the specific micro-finned helically coiled tubes are obtained.
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Heat transfer and pressure drop experimental correlations for air-water bubbly flow

  • Authors: Cui, Wenzhi , Li, Longjian , Chen, Qinghua , Liao, Quan , Jen, Tien-Chien
  • Date: 2006
  • Subjects: Bubbly flow , Gas-liquid flow , Heat transfer , Pressure drop
  • Type: Article
  • Identifier: uj:5268 , http://hdl.handle.net/10210/14937
  • Description: In this paper, a novel air–water bubbly flow heat transfer experiment is performed to investigate the characteristics of pressure drop of airflow and heat transfer between water and tubes for its potential application in evaporative cooling. The attempts to reduce the pressure drop while maintaining higher heat transfer coefficient have been achieved by decreasing the bubble layer thickness through the water pump circulation. Pressure drops of air passing through the sieve plate and the bubbling layer are measured for different height of bubble layer, hole–plate area ratio of the sieve plate and the superficial air velocity. Experimental data show that the increase of bubble layer height and air velocity both increase the pressure drop while the effect of the hole–plate area ratio of the sieve plate on the heat transfer coefficient is relatively sophisticated. A pressure drop correlation including the effects of all the tested parameters is proposed, which has a mean absolute deviation of 14.5% to that of the experimental data. Heat transfer coefficients of the water and the outside tube wall are measured and the effects of superficial air velocity, heat flux and bubble layer height are also examined. Through a dimensional analysis, a heat transfer correlation with a mean absolute deviation of 9.7% is obtained based on experimental data.
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Thermal analysis of the grinding process

  • Authors: Gu, R. J. , Shillor, M. , Barber, G. C. , Jen, T.
  • Date: 2004
  • Subjects: Grinding , Heat transfer
  • Type: Article
  • Identifier: uj:5262 , http://hdl.handle.net/10210/14931
  • Description: A two-dimensional mathematical model for the thermal aspects of a grinding process is presented. The model includes heat conduction in the grinding wheel, workpiece, and coolant. The heat generation through friction, heat loss to the environment as well as debris, and the interaction among the three components are described in detail. A finite-element algorithm is implemented to solve the nonlinear problem. Numerical results, such as temperatures in the grinding wheel and workpiece, are presented.
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Transient heat transfer analysis on a heat pipe with experimental validation

  • Authors: Gutierrez, Gustavo , Catano, Juan , Jen, Tien-Chien , Liao, Quan
  • Date: 2006
  • Subjects: Numerical analysis , Heat transfer , Heat pipes
  • Type: Article
  • Identifier: uj:5270 , http://hdl.handle.net/10210/14939
  • Description: In this study, a transient analysis of the performance of a heat pipe with a wick structure is performed. A complete formulation of the equation governing the operation of a heat pipe during transient conditions are presented and discussed. For the vapor flow, the conventional Navier-Stokes equations are used. For the liquid flow in the wick structure, which is modeled as a porous media, volume averaged Navier-Stokes equations are adopted. The energy equation is solved for the solid wall and wick structure of the heat pipe. The energy and momentum equations are coupled through the heat flux at the liquid-vapor interface that defines the suction and blowing velocities for the liquid and vapor flow. The evolution of the vaporliquid interface temperature is coupled through the heat flux at this interface that defines the mass flux to the vapor and the new saturation conditions to maintain a fully saturated vapor at all time. A control volume approach is used in the development of the numerical scheme. A parametric study is conducted to study the effect of different parameters that affect the thermal performance of the heat pipe. And experimental setup is developed and numerical res ults are validated with experimental data. The results of this study will be useful for the heat pipe design and implementation in processes that are essentially transient.
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Laminar heat transfer and fluid flow in the entrance region of a rotating duct with rectangular cross section : the effect of aspect ratio

A variable heat flux model of heat transfer in grinding : model development

Turbulent heat transfer analysis of a three-dimensional array of perforated fins due to changes in perforation sizes

  • Authors: Jen, T.C , Shaeri, Mohammad Reza , Jen, Tien-Chien
  • Date: 2012
  • Subjects: Heat transfer , Perforated fins
  • Type: Article
  • Identifier: uj:5261 , ISSN 1040-7782 , http://hdl.handle.net/10210/14930
  • Description: Turbulent heat transfer characteristics of three-dimensional and rectangular perforated fins, including perforation like channels along the length of the fins, are investigated. Both dimensions and numbers of perforations are changed at the highest porosity in the study of Shaeri and Yaghoubi [7] to determine the effects of perforation sizes on the heat transfer characteristics of the perforated fins. Results show that at a specific porosity, a fin with a higher number of perforations enhances the heat transfer rate more efficiently. Also, total drag is not only remarkably lower in perforated fins compared with a solid fin, but also becomes smaller by decreasing the number of perforations.
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Convective heat transfer in rotating isothermal ducts

Laminar forced convection in the entrance region of a semi-porous channel

A variable heat flux model of heat transfer in grinding with boiling

Developing fluid flow and heat transfer in a channel partially filled with porous medium

  • Authors: Jen, Tien-Chien , Yan, T. Z.
  • Date: 2005
  • Subjects: Heat transfer , Fluid flow , Porous mediums
  • Type: Article
  • Identifier: uj:5264 , http://hdl.handle.net/10210/14933
  • Description: A three-dimensional computational model is developed to analyze fluid flow in a channel partially filled with porous medium. In order to understand the developing fluid flow and heat transfer mechanisms inside the channel partially filled with porous medium, the conventional Navier–Stokes equations for gas channel, and volume-averaged Navier–Stokes equations for porous medium layer are adopted individually in this study. Conservation of mass, momentum and energy equations are solved numerically in a coupled gas and porous media domain along a channel using the vorticity–velocity method with power law scheme. Detailed development of axial velocity, secondary flow and temperature field at various axial positions in the entrance region are presented. The friction factor and Nusselt number are presented as a function of axial position, and the effects of the size of porous media inside the channel partially filled with porous medium are also analyzed in the present study.
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Thermal aspect of grinding : heat transfer to workpiece, wheel, and fluid

Coupled heat transfer to workpiece, wheel and fluid in grinding, and the occurrence of workpiece burn

Heat transfer augmentation in 3D inner finned helical pipe

Heat transfer augmentation in 3D internally finned and micro-finned helical tube

  • Authors: Li, Longjian , Cui, Wenzhi , Liao, Quan , Mingdao, Xin , Jen, Tien-Chien , Chen, Qinghua
  • Date: 2005
  • Subjects: Helical tubes , Finned tubes , Heat transfer
  • Type: Journal
  • Identifier: uj:5263 , http://hdl.handle.net/10210/14932
  • Description: Experiments are performed to investigate the single-phase flow and flow-boiling heat transfer augmentation in 3D internally finned and micro-finned helical tubes. The tests for single-phase flow heat transfer augmentation are carried out in helical tubes with a curvature of 0.0663 and a length of 1.15 m, and the examined range of the Reynolds number varies from 1000 to 8500. Within the applied range of Reynolds number, compared with the smooth helical tube, the average heat transfer augmentation ratio for the two finned tubes is 71% and 103%, but associated with a flow resistance increase of 90% and 140%, respectively. A higher fin height gives a higher heat transfer rate and a larger friction flow resistance. The tests for flow-boiling heat transfer are carried out in 3D internally micro-finned helical tube with a curvature of 0.0605 and a length of 0.668 m. Compared with that in the smooth helical tube, the boiling heat transfer coefficient in the 3D internally micro-finned helical tube is increased by 40–120% under varied mass flow rate and wall heat flux conditions, meanwhile, the flow resistance is increased by 18–119%, respectively.
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Numerical simulation of fluid flow and heat transfer in a curved square duct by using the Lattice Boltzmann method

  • Authors: Liao, Quan , Jen, T.-C.
  • Date: 2008
  • Subjects: Fluid flow , Heat transfer
  • Type: Article
  • Identifier: uj:5274 , ISSN 1040-7782 , http://hdl.handle.net/10210/14943
  • Description: The study of viscous flow in curved ducts is of fundamental interest in fluid mechanics due to the numerous applications such as flows through turbomachinery blade passages, aircraft intakes, diffusers, heat exchangers, and so on [1–6]. The major effect of curved ducts on the fluid flow involves the strong secondary flow due to the longitudinal curvature in the geometry [7–9]. The presence of longitudinal curvature generates centrifugal force (which is perpendicular to the main flow along the axis) and produces so-called secondary flow on the cross sections of ducts.
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Heat transfer performance in 3D internally finned heat pipe

  • Authors: Liao, Quan , Jen, Tien-Chien , Chen, Qing-hua , Li, Longjian , Cui, Wenzhi
  • Date: 2007
  • Subjects: Heat pipes , Heat transfer , Finned tubes
  • Type: Article
  • Identifier: uj:5276 , http://hdl.handle.net/10210/14945
  • Description: An experimental study of heat transfer performance in 3D internally finned steel-water heat pipe was carried out in this project. All the main parameters that can significantly influence the heat transfer performance of heat pipe, such as working temperature, heat flux, inclination angle, working fluid fill ratio (defined by the evaporation volume), have been examined. Within the experimental conditions (working temperature 40 C–95 C, heat flux 5.0 kw/m2–40 kw/m2, inclination angle 2–90 ), the evaporation and condensation heat transfer coefficients in 3D internally finned heat pipe are found to be increased by 50–100% and 100–200%, respectively, as compared to the smooth gravity-assisted heat pipe under the same conditions. Therefore, it is concluded that the special structures of 3D-fins on the inner wall can significantly reduce the internal thermal resistance of heat pipe and then greatly enhance its heat transfer performance.
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