Experimental investigation of heat transfer enhancement in a circular duct with circumferential fins and circular disks

The characteristics of heat transfer and pressure drop for fully developed turbulent flow in a tube with circumferential fins and circular disks were experimentally studied. The various spacing and sizes of circumferential fins and circular disks were selected as design parameters, while the effects of these parameters on heat transfer enhancement and pressure drop were investigated. In order to quantify the effect of heat transfer enhancement and the increase of pressure drop due to the fins and disks in a tube, the Nusselt numbers and the friction factors for various configurations and operating conditions were compared to those for a corresponding smooth tube. The results showed that the heat transfer rate was significantly enhanced by increasing the height of circumferential fins and decreasing the pitch of circumferential fins. On the other hand, the influence of the disk size and the fin-disk spacing were not significant. Based on the experimental results, a correlation for estimating the Nusselt number was suggested.     

Numerical simulation of natural convection in annuli with internal fins

The solution for the natural convection in internally finned horizontal annuli is obtained by using a numerical simulation of time-dependent and two-dimensional governing equations. The fins existing in annuli influence the flow pattern, temperature distribution and heat transfer rate. The variations of the fin configuration suppress or accelerate the free convective effects compared to those of the smooth tubes. The effects of fin configuration, number of fins and ratio of annulus gap width to the inner cylinder radius on the fluid flow and heat transfer in annuli are demonstrated by the distribution of the velocity vector, isotherms and streamlines. The governing equations are solved efficiently by using a parallel implementation. The technique is adopted for reduction of the computation cost. The parallelization is performed with the domain decomposition technique and message passing between sub-domains on the basis of the MPI library. The results from parallel computation reveal in consistency with those of the sequential program. Moreover, the speed-up ratio shows linearity with the number of processor.     

Squeeze films in porous circular disks

The squeeze film behaviour between two circular disks is analysed. The governing equations are derived when one disk has a porous facing. The fluid in the film region satisfies the modified Reynolds equation and the flow in the porous facing. satisfies the Laplace equation. These equations are solved in closed form and expressions are derived for pressure distribution, load capacity and time of approach for the plates. It is found that with a suitable value of permeability for the porous facing, the pressure distribution becomes more even leading to uniform wear of the entire facing; however, increasing permeability has an adverse effect on the load capacity and time of approach. Numerical data, useful in the design of clutch plates with a porous lining, is given in tabular form.     

Forced convection in a circular pipe with a partially filled porous medium

A study of forced convection in a circular pipe with a partially filled porous medium was numerically investigated. The Brinkman-Forchheimer extension of the Darcy model was used to analyze the and temperature distribution in the porous medium. Our study includes two types of porous layer configurations : (1) a layer attached at the tube wall extending inward towards the centerline and (2) a layer at the centerline extending outward. The effect of several parameters, such as Darcy number, effective viscosity, effective thermal conductivity, and inertia parameter, as well as the effect of geometric parameters, were investigated.     

Combined heat transfer of radiation and forced convection flow of participating gases in a three-dimensional recess

This research work presents a numerical investigation of three-dimensional combined convection-radiation heat transfer over a recess including two inclined steps in a horizontal duct. To simulate the inclined surface boundaries, the blocked off method is employed for both fluid mechanic and radiation problems. The fluid is treated as a gray, absorbing, emitting and scattering medium. In numerical solution of the governing equations including conservation of mass, momentum and energy, the three-dimensional Cartesian coordinate system is used. These equations are solved numerically using the CFD techniques to obtain the temperature and velocity fields. Discretized forms of the governing equations are obtained by the finite volume method and solved using the SIMPLE algorithm. Since the gas is considered as a radiating medium, all of the convection, conduction and radiation terms are presented in the energy equation. For computation of radiative term in energy equation, the radiative transfer equation (RTE) is solved numerically by the discrete ordinates method (DOM) to find the divergence of radiative heat flux distribution inside the radiating medium. The effects of radiation-conduction parameter, optical thickness and albedo coefficient on heat transfer behavior of the system are presented. Comparison of numerical results with the available data published in open literature shows a good agreement.     

横圆管表面空气自然对流换热测试系统开发

为了更好地研究受热横圆管在大空间中的自然对流换热现象,设计开发了横圆管表面空气自然对流换热测试系统.该测试系统采用上下位机模式,可实现横圆管的自然对流换热系数的测定和准则关联式的验证.实际应用结果表明,该测试系统具有功能完善、测量精度高、重复性好等优点,很好地满足了自然对流换热测试的要求.     

A numerical study on mixed convection in a lid-driven cavity with a circular cylinder

A two-dimensional numerical simulation is carried out in this study to investigate mixed convection in a lid-driven cavity with an isothermal circular cylinder. The simulation is conducted at three Reynolds numbers of Re = 100, 500, and 1000 under a fixed Grashof number of Gr = 10⁵. The top wall of the cavity moves to the right at a constant velocity and is kept at a low temperature of T _c , whereas the stationary bottom wall is kept at a constant high temperature of T _h . The immersed-boundary method, which is based on the finite volume method, is adopted for the boundary of the circular cylinder that is present in the square cavity. The present study aims to investigate the effects of circular cylinder on fluid flow and heat transfer in a cavity at different locations. The fluid flow and heat transfer characteristics in the cavity strongly depend on the position of the circular cylinder as well as on the relative magnitude of the forced convection and the natural convection caused by the movement in the top wall of the cavity and the heating at the hot bottom wall, respectively.     

Hydromagnetic squeeze film behavior in porous circular disks

An analysis is presented of the effect of a uniform transverse magnetic field on the squeeze film behavior when a circular disk with a porous facing approaches another disk with uniform velocity. Results for the pressure distribution, load-carrying capacity and film thickness are given as functions of time.     

The effect of serrated fins on the flow around a circular cylinder

An experimental study is performed to investigate the characteristics of near wake flow behind a circular cylinder with serrated fins using a constant temperature anemometer and flow visualization. Various vortex shedding modes are observed. Fin height and pitch are closely related to the vortex shedding frequency after a certain transient Reynolds number. The through-velocity across the fins decreases with increasing fin height and decreasing fin pitch. Vortex shedding is affected strongly by the velocity distribution just on top of the finned tube. The weaker gradient of velocity distribution is shown as increasing the freestream velocity and the fin height, while decreasing the fin pitch. The weaker velocity gradient delays the entrainment flow and weakens its strength. As a result of this phenomenon, vortex shedding is decreased. The effective diameter is defined as a virtual circular cylinder diameter taking into account the volume of fins, while the hydraulic diameter is proposed to cover the effect of friction by the fin surfaces. The Strouhal number based upon the effective diameters seems to correlate well with that of a circular cylinder without fins. After a certain transient Reynolds number, the trend of the Strouhal number can be estimated by checking the ratio of effective diameter to inner diameter. The normalized velocity and turbulent intensity distributions with the hydraulic diameter exhibit the best correlation with the circular cylinder’s data.     

The hydromagnetic squeeze film between porous circular disks with velocity slip

The effect of velocity slip on the behaviour of a squeeze film between two circular disks when a uniform magnetic field is applied between them was studied. The upper disk which has a porous facing backed by a solid housing approaches the lower disk with a uniform velocity. Results are presented for the pressure distribution, the load-carrying capacity and the film thickness as a function of time. The load-carrying capacity decreases as the permeability parameter increases and decreases further as the slip parameter increases.     

Experimental study on natural convection heat transfer from horizontal cylinders with longitudinal plate fins

The purpose of this study was to investigate the effect of various fin numbers, fin heights, and base temperatures on natural convection from horizontal cylinders with longitudinal plate fins experimentally. Experimental results were used to establish a correlation for estimating the Nusselt number under the following conditions: Rayleigh number, 300000–1000000; fin-height-to-cylinder-diameter ratio, 1/6–1/2; and fin number, 9–72. In addition, a contour map was developed to describe the thermal resistance as a function of the fin number and fin thickness.     

Viscous shear effects on the squeeze-film behavior in porous circular disks

On the basis of the Brinkman model (BM), the main objective of this paper is to predict the viscous shear effects on the squeeze-film characteristics between porous circular disks. According to the results obtained, the influences of viscous shear stresses on the squeeze-film behavior are significant and not negligible. It is found that the BM predicts quite different squeeze-film action to those derived by using the Slip-flow model (SFM) and Darcy model (DM). When comparing with that of the SFM, the viscous shear effects of the BM provide an enhancement in the load-carrying capacity and, then, increase the response time of the squeeze-film behavior; but these trends are reversed as compared to that derived by using the DM.     

Numerical investigations of solidification around a circular cylinder under forced convection

Journal of Mechanical Science and Technology - We present numerical investigations of solidification around a cooled circular cylinder in the presence of forced convection. The numerical method is...     

Contact pressure distribution in circular tube expansion using a conical plug

The technique of evaluating contact pressure distributions along a plug metal interface by measuring the deformations in terms of plug surface strains using electrical resistance strain gauges has been explored. To relate the unknown pressure distributions to the strain gauge readings, a finite element model of a plug was used assuming that the normal and shear pressures can be represented as a linear combination of generalized distributions, which have the form of polynomials up to the fourth order.This technique has been suitably applied to a thin brass tube expanding process for a single expansion ratio, and a single conical hollow plug with and without lubricant. To ensure good accuracy in the results, particular care was given to technological plug design and the choice of a number of appropriate strain gauged points on the inside surface of the plug.The more adequate polynomial representations of the normal and shear contact pressures were found by analyzing the degree of randomness in the distribution of the residuals. A comparison was made between experimental pressure distributions and those predicted by the slab method. Results obtained were compared with those derived from the use of a polynomial curve fitting method, besides assuming an average coefficient of friction at the interface.     

Effect of circular cylinder location on three-dimensional natural convection in a cubical enclosure

This paper presents the results of immersed boundary method-based three-dimension numerical simulations of natural convection in a cubical enclosure with an inner circular cylinder at a Prandtl number of 0.7. This simulation spans three decades of Rayleigh number, Ra, from 10³ to 10⁶. The location of the inner circular cylinder is changed vertically along the centerline of the cubical enclosure. This study primarily focuses on the effects of both buoyancy-induced convection and the location of the inner circular cylinder on heat transfer and fluid flow in the cubical enclosure. In the range of Rayleigh numbers considered in this study, the thermal and flow fields eventually reach steady state, regardless of the location of the inner cylinder. When Ra is 10³, the end wall of the cubical enclosure has a negligible effect on the thermal and flow fields in the enclosure. However, in the range of 10⁴ ≤ Ra ≤ 10⁶, the effect of the end wall on heat transfer and fluid flow in the enclosure depends on both the location of the inner cylinder and the Rayleigh number. Detailed analysis results for the distribution of streamlines, isotherms, and Nusselt numbers are presented in this paper.     

Heat transfer characteristics of a short helical plate in a horizontal circular tube

Journal of Mechanical Science and Technology - Enhancement techniques based on artificial roughness are used in numerous applications of heat exchangers. Heat exchange devices are essential...     

Squeeze film characteristics of parallel circular disks lubricated by ferrofluids with non-Newtonian couple stresses

The parallel circular squeeze film disks with a non-Newtonian ferrofluid in the presence of transverse magnetic fields are investigated in this paper. A modified Reynolds equation is derived by applying the Shliomis ferrohydrodynamic model incorporating the Stokes microcontinuum theory. Some special cases can be recovered from the present study. Comparing with the Newtonian non-ferrofluid case, the non-Newtonian ferrofluid lubricated squeeze films provide a higher load capacity and lengthen the approaching time. Some numerical results of the load capacity for various parameters are also included in a table for engineering applications.     

玻璃圆管中圆形截面杆柱位置的测定

杆柱在井眼中的屈曲是石油工业中的常见现象,屈曲的形态直接影响杆柱的强度。为了从实验中分析屈曲的形态,考虑受压的杆柱在玻璃圆管中的形变测量问题。应用针孔相机模型,并考虑玻璃对光的折射现象,从光线的传播路径分析了一个断面上实际玻璃圆管中杆柱断面中心与照片边缘之间的几何关系,推导出由杆柱像的边缘得出通过实际杆柱断面中心的直线方程。结合照片边缘检测方法,可由两幅不同角度的照片,得到实际杆柱断面中心的位置。该公式和方法可用于玻璃管约束的杆柱后屈曲构型测定的实验研究。     

A numerical study on the conjugate natural convection in a circular pipe containing water

In this paper, the effect of material property of pipe on the conjugate natural convection in a circular pipe containing water was investigated by solving the unsteady incompressible Navier-Stokes equations coupled with energy equations of the water and pipe. Natural convection and conduction of water inside the pipe was coupled with the conduction of the pipe whose bottom was subject to uniform heat source. From the present grid resolution and time-step independent solutions, it has been confirmed that the water temperature inside a PVC pipe was higher than that inside a steel pipe due to the smaller heat capacity of PVC and that the streamline patterns of the two cases were found to be opposite because the thermal diffusivity of steel (PVC) is larger (smaller) than that of water such that steel (PVC) pipe is heated faster (slower) than water. Furthermore, a quantitative comparison of heat flux to water was performed by examining the distributions of the heat flux along the inside walls of steel/PVC. The average temperature of water inside steel was found to be higher than that inside PVC at the initial stage of heating. On the other hand, PVC provided a larger heat flux to water when it reached a steady value.     

缩放管内脉动流动对流换热的数值模拟研究

通过数值模拟计算,研究在不同脉动频率、脉动振幅下的管内流体脉动对缩放管的传热和阻力的影响。研究结果表明:管内流体脉动能够强化缩放管的传热效果,相比稳态时,传热被强化了约为16%;管内流体脉动在一定条件下,也会增大缩放管的沿程阻力。