Heat transfer enhancement in dimpled tubes

Heat transfer enhancement was investigated in a coaxial-pipe heat exchanger using dimples as the heat transfer modification on the inner tube. Tube-side Reynolds numbers were in the range of 7.5×10³–5.2×10⁴ for water flow. A constant annular mass flow rate was chosen to obtain the highest possible Reynolds number of 1.1×10⁴. Typically, the heating water inlet temperature was 68.1±0.1^∘C.All six variants with inward-facing, raised dimples on the inner tube increased the values of heat transfer coefficient significantly above those for the smooth tube. Heat transfer enhancement ranged from 25% to 137% at constant Reynolds number, and from 15% to 84% at constant pumping power. At a constant Reynolds number, the relative J factor (ratio of heat transfer coefficient to friction factor, relative to smooth tube values), had values from 0.93 to 1.16, with four dimpled tube configurations having values larger than unity. Despite the extremely simple design, this outperforms almost all heat transfer enhancements recommended in the literature. A correlation based on the results of the present work appears to be sufficiently accurate for predicting heat transfer coefficients and friction factors for the design of dimpled-tube heat exchangers.     

NUMERICAL MODELING OF HELICAL FLOW OF PSEUDOPLASTIC FLUIDS

The laminar helical flow of non-Newtonian pseudoplastic fluids in concentric and eccentric annuli with a rotating inner cylinder has been investigated numerically. A finite volume algorithm with a nonstaggered grid system is used to analyze the problem. A nonorthogonal curvilinear coordinate system is employed to handle the irregular geometry of aneccentric annulus. The power-law constitutive equation is used to model the shear rate dependent viscosity of a pseudoplastic fluid. The computer code is validated against an available analytical solution for helical flow in a concentric annulus. It is observed that for a certain axial pressure gradient the axial flow rate increases within creasing rotational speed of the inner cylinder. The torque needed to rotate the inner cylinder decreases with increasing axial pressure gradient. These are explained in terms of the shear-thinning effect of a pseudoplastic fluid. The discharge as well as torque are found to increase with increasing eccentricity. The flow field in an eccentric annulus is complex in nature since vigorous secondary flow is produced in addition to the primary axial helical flow. The location and extent of the secondary flow is studied and theresults are presented for various eccentricities. The results will be useful in planning oil and gas well drilling operations.     

Application of the semi-analytical cavitation model to flows in a centrifugal pump

Visualization experiments are carried out to investigate the cavitating flows in a centrifugal pump at different flow rates. The cavity lengths under different pump inlet pressure are obtained. The semi-analytical cavitation model is improved since it is not suitable for predicting large cavities full of vapor. The improved cavitation model is used to numerically study the steady and rotating cavitation in the centrifugal pump. Compared with the Zwart cavitation model, the numerical results predicted by the semi-analytical model agree much better with the experiments, especially for large cavities. The cavity lengths at the suction side are overestimated during the simulations, especially by using the Zwart model. At low flow rates, the prediction of the rotating cavitation effect is weaker and the cloud shedding frequency is smaller than those in the experiments.     

Study of magnetohydrodynamic convection between two rotating cylinders filled with casson fluid and the viscous dissipation effect

We have studied the forced convection of a viscous incompressible and electrically conducting non‐Newtonian Casson fluid between two rotating cylinders with viscous dissipation effect. An angular velocity and gives to inner and outer cylinders, respectively, and constant heat flux presented on the inner cylinder surface. Also, the outer cylinder is taken as insulated. Here, we have discussed three cases: (i) The inner cylinder is rotating with a constant angular velocity while the outer cylinder is at rest; (ii) Both the cylinders rotate in the identical direction with equal angular velocity; and (iii) Both the cylinders are rotating with equal angular speed but the outer cylinder rotates in the opposite direction of the inner cylinder. The governing equations are solved by numerical techniques using MATLAB Software and the results are obtained graphically.     

Heat transfer on a horizontal rotating cylinder near a flat plate in a cross‐flow

An experimental study of heat transfer on a horizontal rotating cylinder near a flat plate was performed. The cylinder and plate were set in a cross‐flow. Temperature distribution and coefficients of local heat transfer were measured by a Mach–Zehnder interferometer. Flow visualization was made using smoke. Rotating Reynolds numbers (Re_r) and cross‐flow Reynolds numbers (Re_d) were varied from 0 to 2000. The spaces between cylinder and plate were varied from 1 × 10^?3 m to 5 × 10^?3 m. The rotating direction of cylinder was changed clockwise or counterclockwise. The following results are obtained: When the space between the rotating cylinder and flat plate is the same as the displacement thickness on the plate, the heat transfer on the cylinder near the plate has the best performance. We have procured the empirical equation of heat transfer from a rotating cylinder near the flat plate in the cross‐flow. 8 2010 Wiley Periodicals, Inc. Heat Trans Asian Res; Published online in Wiley Online Library ( wileyonlinelibrary.com ). DOI 10.1002/htj.20329     

Lattice Boltzmann simulation of natural convection heat transfer in an elliptical-triangular annulus

A numerical study for steady-state, laminar natural convection in a horizontal annulus between a heated triangular inner cylinder and cold elliptical outer cylinder was investigated using lattice Boltzmann method. Both inner and outer surfaces are maintained at the constant temperature and air is the working fluid. Study is carried out for Rayleigh numbers ranging from 1.0 × 10³ to 5.0 × 10⁵. The effects of different aspect ratios and elliptical cylinder orientation were studied at different Rayleigh numbers. The local and average Nusselt numbers and percentage of increment heat transfer rate were presented. The average Nusselt number was correlated. The results show that by decreasing the value of aspect ratio and/or increasing the Rayleigh number, the Nusselt number increases. Also the heat transfer rate increases when the ellipse positioned vertically.     

The Effect of Viscous Dissipation on Heat Transfer in Cavities of Varying Shape Due to an Inner Rotating Circular Cylinder

In this study, the finite-volume-based computational methodology was used to investigate the effect of a highly viscous fluid on the evolution of flow and thermal fields in cavities of different shapes, including a circle, square, and equilateral triangle, due to an inner isothermal rotating cylinder. The temperature of the cavity wall was kept constant, but differed from that of the inner cylinder. Numerical results revealed that the triangular cavity had the greatest ability to dissipate internal thermal energy through the side walls, while the circular cavity had the worst performance.     

Numerical Simulations of Convective Heat Transfer in Rayleigh-Bénard Convection and a Rotating Annulus

ABSTRACT

Numerical studies of fluid and heat flows in a sealed rotating annulus having a heated outer cylinder and a cooled inner cylinder are performed. Flow is induced by buoyancy in the centrifugal field and affected by the Coriolis force. We focus on the heat transfer efficiencies characterized by correlations of Nusselt number with Rayleigh number ( Nu ～ Ra^γ); however, the flow structures are also discussed. We begin with a discussion of recent progress in Rayleigh-Bénard convection. Its connection to the rotating annulus systems used in the current study are then described. We hope to present a clearer picture of free-convective heat transfer in enclosures by comparing and interpreting results from various systems. Typical values for γ vary between 1/5 and 1/2 inclusive.     

Natural convection heat transfer of high temperature gas in an annulus between two vertical concentric cylinders

Water cooling panels have been adopted as the vessel cooling system of the High Temperature Engineering Test Reactor (HTTR) to cool the reactor core indirectly by natural convection and thermal radiation. In order to investigate the heat transfer characteristics of high temperature gas in a vertical annular space between the reactor pressure vessel and cooling panels of the HTTR, we carried out experiments and numerical analyses on natural convection heat transfer coupled with thermal radiation heat transfer in an annulus between two vertical concentric cylinders with the inner cylinder heated and the outer cylinder cooled. In the present experiments, Rayleigh number based on the height of the annulus ranged from 2.0 × 10⁷ to 5.4 × 10⁷ for helium gas and from 1.2 × 10⁹ to 3.5 × 10⁹ for nitrogen gas. The numerical results were in good agreement with the experimental ones regarding the surface temperatures of the heating and cooling walls. As a result of the experiments and the numerical analyses, the heat transfer coefficient of natural convection coupled with thermal radiation was obtained as functions of Rayleigh number, radius ratio, and the temperatures and emissivities of the heating and cooling wall surfaces. © 2005 Wiley Periodicals, Inc. Heat Trans Asian Res, 34(5): 293–308, 2005; Published online in Wiley InterScience ( www.interscience.wiley.com ). DOI 10.1002/htj.20070     

Pressure Loss Through Sharp 180 deg Turn in a Relatively Short Two－Pass Smooth and Rib－Roughened Channel

Ｐｒｅｓｓｕｒｅ　Ｌｏｓｓ　Ｔｈｒｏｕｇｈ　Ｓｈａｒｐ　１８０　ｄｅｇ　Ｔｕｒｎ　ｉｎ　ａ　Ｒｅｌａｔｉｖｅｌｙ　Ｓｈｏｒｔ　Ｔｗｏ－Ｐａｓｓ　Ｓｍｏｏｔｈ　ａｎｄ　Ｒｉｂ－Ｒｏｕｇｈｅｎｅｄ　ＣｈａｎｎｅｌＰｒｅｓｓｕｒｅＬｏｓｓＴｈｒｏｕｇｈＳｈ...     

Nonlinear stability analysis of thin condensate falling film inside a rotating vertical cylinder

This paper investigates the stability of thin condensate film flowing down on the inner surface of a rotating vertical cylinder by means of long wave perturbation method in a two-step procedure. In the first step, the normal mode method is used to characterize the linear behavior. In the second step, an elaborated nonlinear film flow model is solved by using the method of multiple scales to characterize flow behavior at various states of sub-critical stability, sub-critical instability, supercritical stability, and supercritical explosion. The procedure follows the previous research [C.I. Chen, C.K. Chen, Y.T. Yang, Int. J. Heat Mass Transfer 47 (2004) 1937–1951] which concerns with the thin condensate falling film on the outer surface of a rotating vertical cylinder. The modeling results indicate that by increasing the rotation speed, Ω, and the radius of cylinder, R, the condensate film becomes more stable, which is totally opposite to the previous study.     

Impact of relative motion of a magnetic field on unsteady magnetohydrodynamic natural convection flow with a constant heat source/sink

This study analyzes time‐dependent magnetohydrodynamics natural convective flow of a viscous incompressible fluid in an annulus with ramped motion of the boundaries. The governing momentum and energy equations are solved analytically, in terms of the modified Bessel function of the first and second kinds. The influence of governing parameters such as the Hartman number, radius ratio, Grashof number, heat absorption parameter, and Prandtl number are discussed with the help of line graphs. It is found that the Hartmann number has a retarding effect on fluid velocity when K = 0.0 and K = 0.5, while the reverse effect is noticed when K = 1.0. The Hartman number also decreases the mass flow rate for all cases of $K$ while it enhances the skin friction at the inner surface of the outer cylinder. It increases the skin friction at the outer surface of the inner cylinder when K = 1.0 and K = 0.0, but decreases the skin friction at the outer surface of the inner cylinder when K = 0.5.     

Heat transfer regulation in a rectangular enclosure using a rotating plate

The effects of an inner rotating plate with horizontal axis on the heat transfer in a differentially heated vertical enclosure were investigated experimentally. The aspect ratio of the enclosure height/width was 1 throughout the experiments. An acrylic plate with a small thermal conductivity was installed horizontally at the center of the square enclosure, and was rotated at various speeds for normal and reverse rotations by using the motor attached outside of the enclosure. Purified water was used as the working fluid. The flow pattern was sketched by a visualization experiment using aluminum powder. The heat transfer results were also compared with those from a previous paper on a rotating cylinder. It is clarified here that the heat transfer rate of the enclosure depends largely on the parameter Gr_w/Re_ω², and is characterized by three regions. The heat transfer rate of the enclosure with a rotating plate is somewhat larger than that of a rotating cylinder in the forced convection region. The rotating plate used here will be useful for regulation of wide‐ranging heat transfer. © 2003 Wiley Periodicals, Inc. Heat Trans Asian Res, 32(4): 342–353, 2003; Published online in Wiley InterScience ( www.interscience.wiley.com ). DOI 10.1002/htj.10099     

A numerical study of natural convection in a square enclosure with a circular cylinder at different vertical locations

Numerical calculations are carried out for natural convection induced by a temperature difference between a cold outer square enclosure and a hot inner circular cylinder. A two-dimensional solution for unsteady natural convection is obtained, using the immersed boundary method (IBM) to model an inner circular cylinder based on the finite volume method for different Rayleigh numbers varying over the range of 10³–10⁶. The study goes further to investigate the effect of the inner cylinder location on the heat transfer and fluid flow. The location of the inner circular cylinder is changed vertically along the center-line of square enclosure. The number, size and formation of the cell strongly depend on the Rayleigh number and the position of the inner circular cylinder. The changes in heat transfer quantities have also been presented.     

Turbulent Mixed Convection—Radiation Interactions in a Cavity

Two-dimensional turbulent mixed convection-surface radiation interaction phenomenon in a back-wall-heated open square cavity is numerically investigated. The flow medium is air, and the turbulence model used is the Low-Reynolds-Number κ-ε Scheme. Calculations have been performed for Grashof numbers Gr up to 10~(10). And the Richardson number Ri covers a range of 4×10~(-3)-45. It is shown that within a rather extensive range of Ri, the effects of radiation on the heat transfer and fluid flow in the cavity are significant, should not be neglected and become stronger with the increase of Ri and Gr.     

Measurement of Thermal Expansion Coefficients with Holographic Technique

ＭｅａｓｕｒｅｍｅｎｔｏｆＴｈｅｒｍａｌＥｘｐａｎｓｉｏｎＣｏｅｆｆｉｃｉｅｎｔｓｗｉｔｈＨｏｌｏｇｒａｐｈｉｃＴｅｃｈｎｉｑｕｅ￥ＺｈｉｆｅｎｇＺｈａｎｇ（ＤｅｐａｒｔｍｅｎｔｏｆＭｅｃｈａｎｉｃａｌａｎｄＥｎｖｉｒｏｎｍｅｎｔａｌＥｎｇｉｎｅｅｒ...     

Analysis of a novel rotating disk cylinder engine concept for power generation

A new type of engine concept is proposed, namely a rotating disk cylinder engine. Unique secondary rotors with grooves act as guide vanes for a working fluid to power a main rotor with multiple cylindrical pistons. Currently, no literature exists on the proposed device and the modelling thereof. The aim of this paper is to determine the dynamic and thermodynamic behaviour of the machine so that recommendations can be made for further research work. A transient as well as a steady‐state thermodynamic model is developed for an adiabatic expansion process using first and second law analysis with air as ideal gas. The model is compared with experimental results of a piston engine driven by compressed air. Results show that the secondary rotor's angle of rotation is not a linear function of the main rotor's angle of rotation, which suggests a limitation in terms of operating speed. Furthermore, for constant power, the efficiency increases as the inlet pressure and speed decrease, while for constant efficiency, the power increases as the inlet pressure and speed increase. Results show the advantage of using three secondary rotors as it drastically decreases the pressure requirement for a constant torque output. The speed and inlet pressure of the device is, however, limited by the mass and material properties of the rotors as well as the effects of leakage. These limitations have not been considered in this initial analysis of the device and are recommended for future work.     

A numerical study of turbulent flow and conjugate heat transfer in concentric annuli with moving inner rod

A numerical study is conducted to investigate turbulent flow and conjugate heat transfer in a concentric annulus with a heated inner cylinder moving in the streamwise direction. A modified two-equation k-ε model with low Reynolds number treatment near wall is employed to model the Reynolds stress and turbulent thermal field which are based on Boussinesq’s approximation. The governing equations are numerically resolved by means of a hybrid finite analysis method. A uniform inlet flow and thermal conditions are specified to consider the effects of entrance of both solid and fluid regions. For a constant Prandtl number of 6.99 of water flow, calculating results of the time-averaged streamwise velocity, turbulent viscosity and temperature field are obtained for the Reynolds numbers from 1.0 × 10⁴ to 5.0 × 10⁵, rod velocity ratio between 0 and 1.0, and the radius ratio ranging from 0.286 to 0.750. The parametric studies show that the bigger rod speed ratio or the radius ratio is, the temperature is higher within solid rod. For a certain absolute rod speed, temperature profile diminishes at both sides of solid rod and fluid as Reynolds number grows. Numerical results also show that compared with the case of β=0 where solid rod is stationary, for large rod speed ratio the averaged axial velocity and turbulent viscosity profiles have substantial deformations, that is, the gradient of averaged axial velocity and turbulent viscosity near rod surface greatly reduced by the axial movement of solid rod.     

Influences of a confined elliptic cylinder at different aspect ratios and inclinations on the laminar natural and mixed convection flows

Numerical investigations were carried out for natural and mixed convection within domains with stationary and rotating complex geometry by using an immersed-boundary method. The method was first validated with flows induced by natural convection in the annulus between concentric circular cylinder and square enclosure, and the grid-function convergence tests were also examined. Natural convection induced by isothermally elliptic cylinder was further investigated for different Rayleigh numbers within the range of 10⁴–10⁶ and the influence of the outer enclosure was also considered. The parameters investigated in the study included Rayleigh number, axis ratio and inclination angle of the elliptic cross-section. Local and average heat transfer characteristics were fully studied around the surfaces of both inner cylinder and outer enclosure. Finally, mixed convection in a square enclosure with an active rotating elliptic cylinder was considered and the heat transfer quantities of the system were obtained for different rotating speeds.     

Approximate evaluation of operating conditions and effectiveness of solar impurity thermovoltaic element

The effectiveness of solar impurity thermovoltaic elements based on semiconductors with forbidden gap width 0.3 eV with thin highly doped p- and n-layers with concentration ∼5 × 10¹⁹ cm⁻³, containing in addition a deep impurity with concentration ∼4 × 10¹⁸ cm⁻³ and ionization energy 0.15 eV, is shown to reach 30–40% with solar radiation intensity 0.1 W/cm².