Convective heat transfer on a rotating finned cylinder with transverse airflow

The present experimental investigation relates to the convective heat transfer determination around annular fins mounted on a rotating cylinder with air crossflow. The mean convective heat transfer coefficient can be identified by solving the inverse conduction heat transfer problem during the fin cooling process. We used an inverse method, based on the mean squared error, to develop a model of mean convective heat transfer, taking lateral conduction into account. Tests were carried out for rotational Reynolds numbers Re_ω between 2150 and 17,200, air crossflow Reynolds numbers Re_U between 0 and 39,600, and fin spacings u in the range 10 mm to ∞, u = ∞ corresponding to the single disk case. For each fin spacing, the relative influences of the rotational and airflow forced convections on the heat transfer were analyzed and correlations of the mean Nusselt number on the fin, relative to both Reynolds numbers, are proposed. Moreover, an efficiency definition, that allows optimal geometrical configurations of the finned cylinder to be identified for the given operating conditions, is proposed.     

不同材质花瓣形扁通道内翅换热管的实验研究

通过实验的方法研究了一种花瓣形扁通道内翅换热管的对流换热和阻力特性,拟合了所测Re范围内对流换热和阻力实验关联式,并运用相同质量流量、相同泵功率和相同阻力降这3种准则比较了采用不同材质时翅片管与普通光管之间的传热效果。结果表明,翅片材质对换热强化效果有较大影响,无论采用哪种材质,花瓣形扁通道内翅管均有较强的换热效果,特别是在低盅条件下,强化效果更加明显。     

Convective heat transfer inside a rotating cylinder with an axial air flow

This article presents an experimental identification technique for the convective heat transfer coefficient inside a rotating cylinder with an axial airflow. The method consists in heating the outer face of the cylinder using infrared lamps, and acquiring the evolution of the external surface temperature versus time using an infrared camera. Heat transfer coefficients are identified via three methods. The first one is based on an inverse model, the second one assumes the wall of the cylinder as a thermally thin wall and the third one is based on an analytical method permitting to obtain the temperature field within the whole cylinder. The experiments were carried out for a rotational speed ranging from 4 to 880 rpm corresponding to rotational Reynolds numbers varying from 1.6×10³ to 4.7×10⁵ and an air flow rate varying from 0 to which corresponds to an axial Reynolds numbers ranging from 0 to 3×10⁴. Correlations connecting the Nusselt number to the axial and rotational Reynolds numbers are also proposed.     

Laminar forced convection from a rotating horizontal cylinder in cross flow

The influence of non-dimensional rotational velocity, flow Reynolds number and Prandtl number of the fluid on laminar forced convection from a rotating horizontal cylinder subject to constant heat flux boundary condition is numerically investigated. The numerical simulations have been conducted using commercial Computational Fluid Dynamics package CFX available in ANSYS Workbench 14. Results are presented for the non-dimensional rotational velocity α ranging from 0 to 4, flow Reynolds number from 25 to 40 and Prandtl number of the fluid from 0.7 to 5.4. The rotational effects results in reduction in heat transfer compared to heat transfer from stationary heated cylinder due to thickening of boundary layer as consequence of the rotation of the cylinder. Heat transfer rate increases with increase in Prandtl number of the fluid.     

Forced convection on grey cast iron plate-fins: Prediction of the heat transfer coefficient

The main goal of the present work is to evaluate the convective heat transfer coefficient at the surface of grey cast iron plate-fins. A hybrid numerical/experimental approach was adopted, i.e., temperature was measured at selected points at the fin surface and an inverse problem technique based on optimization was used to obtain the heat transfer coefficients. The direct heat transfer problem was solved numerically using the finite volume method, whilst the optimization problem was resolved based on particle swarm optimization (PSO). Firstly, the temperature dependence is investigated by comparing uniform, linear and parabolic equations for the heat transfer coefficient. The hybrid approach was validated through an energy balance applied to the finned surface. The parametric study was performed by assessing the influence of the fin spacing and flow velocity on the convective heat transfer coefficient: the results indicate that the convective coefficient is enhanced with increasing Reynolds number and fin spacing. Finally, the experimental results for the Nusselt number in the parametric study were condensed into a single new empirical correlation with good accuracy.     

Heat transfer study in a rotor–stator system air-gap with an axial inflow

Discoidal rotor–stator systems are nowadays sometimes used in electrical wind generator. The cooling of such a system is a major problem due to the fact that high electrical losses are dissipated for relatively low rotational speed, responsible of the cooling. A new cooling solution is then investigated in this paper. So, this paper presents an experimental study of the local heat transfers on the rotor surface in the air-gap of a discoidal rotor–stator system, in which an air jet comes through the stator and impinges the rotor. To determine the surface temperatures, measurements were taken on the rotor, using an experimental technique based on infrared thermography. A thermal balance equation was used to identify the local convective heat transfer coefficient. The influence of the axial Reynolds number Re_j and the rotational Reynolds number Re was measured and compared with the data available in the literature. Local convective heat transfer coefficients were obtained for an inter-disk dimensionless spacing interval G ranging from 0.01 to 0.16 for Re_j between 0 and 41,666 and for Re between 20,000 and 516,000. The rotating disk can thus be divided into zones: one dominated by the air jet near the center of the rotor and one affected by both the air jet and rotation. Even though these two zones are not located in the same place on the disk, the heat transfers with non-zero impinging jets appear to be continuously improved compared to those with no jets. Critical radii over the rotor surface are identified and correlations are given.     

Convective heat transfer from a rotating cylinder with inline oscillation

In this study convective heat transfer from a rotating cylinder with inline oscillation is studied using a finite element method based on the Characteristic Based Split method (CBS) to solve governing equations consisting of continuity, full Navier–Stokes, and energy equations. Employing the Arbitrary Lagrangian-Eulerian (ALE) formulation, the dynamic unstructured triangular grid used here is accompanied by lineal and torsional spring analogy to consider large boundary movements. Simulations are conducted to study convective heat transfer past a rotating cylinder with inline oscillation at Reynolds numbers of 100, 200 and 300. Different rotational speeds of the cylinder in the range of 0–2.5 are considered at various oscillating amplitudes and frequencies with three different Prandtl numbers of 0.7, 6 and 20. Effects of oscillation and rotation of cylinder on the temperature and flow field, vortex lock-on, mean Nusselt number, and the pattern of vortex shedding are investigated in detail at constant temperature boundary condition on the cylinder surface. It is found that similar to the fixed cylinder, beyond a critical rotating speed, the vortex shedding is strongly suppressed. Furthermore, as the rotational speed of the cylinder increases, both the Nusselt number and the drag coefficient decrease rapidly. In the vortex lock-on region, the Nusselt number increases rapidly.     

Etude des structures convectives et du transfert thermique autour d'un cylindre en deuxième nappe dans une configuration en quinconce,en écoulement d'air chargé de gouttelettes d'eau

We describe an experimental study of the influence of longitudinal and transversal pitch on the convective transfer around a cylinder on the second row of a three cylinder staggered bank, operating in an airstream charged with sprayed water at moderate Reynolds number. The analysis of the wall velocity gradient and its fluctuation shows the effects of geometrical parameters on various characteristic zones of the flow: laminar boundary layer, separation and vortices. The modifications of local Nusselt number evolution compared with the single cylinder are explained. For the disposition which gives the highest heat transfer, a correlation of local Nusselt number is proposed.     

Heat transfer in a small gap between co-axial rotating cylinders

This paper investigates the local heat transfer of a co-axial rotating cylinder. In the inner flow field of the rotating cylinder, the dimensionless parameters include the rotational Reynolds number (Re_Ω) and buoyancy parameter (Gr). The test rig is designed to make the rotating in the inner cylinder and stationary in the outer cylinder. The local temperature distributions of the inner and outer cylinder on axial direction were measured. Under the experimental condition, whereas the ranges of the rotational Reynolds number are 2400 ≤ Re_Ω ≤ 45,000. Experimental results reveal that the rotational Reynolds number's increase is with the heat transfer coefficient distributions increase types. Finally, the local heat transfer rate on the wall are correlated and compared with that in the existing literature.     

Non-linear,unsteady free convection in a vertical cylinder submitted to a horizontal thermal gradient: measurements in water between 6 and 21°C and a theoretical model of convection

The non-linear, unsteady behaviour of water contained in a vertical cylinder of yellow brass when submitted to a horizontal initial thermal gradient is investigated by following the temperature decay in the centre of a cylinder. Experimental results are interpreted by means of a theoretical model which allows the deduction of equations for temperature, velocity, pressure and density in the nucleus. The new equations are compared with those of conduction to provide an evaluation of the convective contribution to heat transfer. Our data indicate that when a characteristic dimensionless group which has the form of a Rayleigh number reaches a critical value of 1600 ± 50, the heat transfer may be described by a pure conduction equation.     

On the evaluation of a finned annular tube in convective heat transfer performance in the presence of Ag/oil nanofluid for a constant thermal flux rate boundary condition

In the present empirical work, the effectiveness of a finned annular tube in the presence of Ag/oil nanofluid is investigated. An annular tube with axial fins was considered as the test case. Suspended Ag nanoparticles in different volume concentrations of 0.011%, 0.044%, and 0.176% were examined in this work. The setup was designed in a way to be sure that the flow is hydrodynamically fully developed along the tube. This experiment has been done in a laminar flow regime in which Reynolds number was less than 160 for all the studied cases. The finned annular tube was wrapped with a coil that satisfied the condition of a constant thermal flux rate of 204 W on the outer boundary. Based on the acquired data, the convective heat transfer coefficient was obtained for all the nanofluid cases and compared to the base fluid. It was observed that the convective heat transfer coefficient substantially rises by increasing the nanoparticles. Which for the best case (volume concentration of 0.171% and Reynolds number of about 160), this factor was about a 33% enhancement compared to the base fluid.     

Heat transfer on a radially rotating heated cylinder

Experiments are conducted to investigate the convective heat transfer on a radially rotating heated cylinder. In the experiment, one uses cold air-hot cylinder instead of hot air-cold blade in a real engine. The hollow bakelite test rotating cylinder is pasted with a heater made of 0.03 mm thin film of stainless steel. The maximum air stream velocity is 20 m/s with the corresponding Reynolds number of 1.2 × 10⁵ that is high enough to simulate the real turbine blade of Re ≈ 10⁵. The rotation-induced cross stream flow affect the heat transfer coefficient on the cylinder surface. The effect is more prominent for the cases with higher rotational speeds and lower Reynolds numbers. Due to rotation, the heat transfer enhancement at lower Reynolds number is greater than those at a higher one.     

Numerical simulation of heat transfer and fluid flow past a rotating isothermal cylinder – A LBM approach

In this paper, a viscous fluid flowing past a rotating isothermal cylinder with heat transfer is studied and simulated numerically by the lattice Boltzmann method (LBM). A numerical strategy for dealing with curved and moving boundaries of second-order accuracy for both velocity and temperature fields is proposed and presented. The numerical strategy and method are validated by comparing the present numerical results of flow without heat transfer with those of available previous theoretical, experimental and numerical studies, showing good agreements. On this basis, the convective heat transfer performance in such rotational boundary environments is further studied and validated; the numerical results are reported in the first time. The effects of the peripheral-to-translating-speed ratio, Reynolds number and Prandtl number on flow and heat transfer are discussed in details.     

Modeling Heat Transfer and Skin Friction Frequency Responses of a Cylinder in Cross-Flow—a Unifying Perspective

The dynamic behavior of skin friction and heat transfer of a cylinder in pulsating cross-flow is investigated. Existing analytical solutions are presented as transfer functions versus frequency, known from control theory. New results for Reynolds number ranges where no appropriate model exists until now are derived. These results are obtained by the combination of computational fluid dynamics and system identification (CFD/SI). In the CFD/SI approach, time series for fluctuations of skin friction and the rate of heat transfer are generated by imposing broad-band inlet velocity fluctuations in a CFD simulation of laminar flow across a cylinder. Direct numerical simulations are conducted for mean flow Reynolds numbers between 0.1 and 40, solving the incompressible Navier–Stokes equations in a two-dimensional domain using a finite volume approach. The SI framework allows to estimate transfer functions for the response of skin friction and heat transfer to velocity fluctuations from time series data. Available analytical models for the dynamic behavior of skin friction and heat transfer usually match the simulated data up to a point, but do not give any dependence on Reynolds number. This shortcoming is addressed in this work. The identified models especially excel at Reynolds numbers of order 10.     

Coupled gas convection and unsteady conduction effects in fluid bed heat transfer based on a single particle model

A self-consistent model of heat transfer in moderate to large particle gas fluidized beds is constructed by combining the convective heat transfer model of Adams and Welty [A.I.Ch.E. Jl25, 395–405 (1979)] with a numerical analysis of transient conduction within the solid particles to obtain the particle convective contribution to the heat transfer. Computations using an ADI finite difference scheme reveal that the particle convective contribution is weakly dependent on Reynolds number but strongly affected by gas and solid thermal conductivity and particle Fourier number. Time-averaged emulsion phase Nusselt numbers computed using the model are compared with available experimental data.     

Heat transfer study in a discoidal system: The influence of rotation and space between disks

This article presents an experimental study of the local heat transfer on the rotor surface in a discoidal rotor–stator system air-gap in which an air jet comes through the stator and impinges the rotor. To determine the surface temperatures, measurements were taken on the rotor, using an experimental technique based on infrared thermography. A thermal balance was used to identify the local convective heat transfer coefficient. The influence of the dimensionless spacing interval G between the disks and of the rotational Reynolds number Re was measured and compared with the data available in bibliography. Local convective heat transfer coefficients were obtained for an axial Reynolds number Re_j = 41.6 × 10³, a rotational Reynolds number Re between 0.2 × 10⁵ and 5.16 × 10⁵, and a dimensionless spacing interval G ranging from 0.01 to 0.16.     

Investigation of the influence of free convection on the heat transfer of cylinders with different aspect ratios

IntroductionThe heat trallsfer from Hot-Wires in low-velocityforced flows can be significantly influenced by buoyancy effects. In Order to quantify this buoyant contribuhon tO the total heat trallsfer, experiments underweighhessness were compared with laboratory measurementS. The ndcrogravity ig measurementS havebeen performed in the DrOP TOwer Bremen whichprovides a reduced gravity level of 10-5 g during aperiod Of 4.7 s. In this work tWo wires with differentaspect ratios F==l/d, where …     

Numerical study of mixed convective heat transfer in a lid‐driven enclosure with rotating cylinders

A numerical simulation is performed to characterize the mixed convective transport in a three‐dimensional square lid‐driven enclosure with two rotating cylinders. The top wall is moving in the positive x‐direction, and the bottom wall is at a higher fixed temperature compared with all other isothermal walls. Both cylinders are rotating in its own plane about their centroidal axis. On the basis of rotation of both cylinders in clockwise or counter‐clockwise directions, four rotational models are studied. Various controlling parameters considered in the present study are Grashof number (10 ³ < Gr < 10 ⁵), rotating speed of the cylinder (5 < ω < 50), and the Reynolds number based on top wall movement is fixed to 100. The effect of cylinder rotation on the heat transfer of bottom wall is reported with the help of streamlines, contour plots of z‐component of vorticity, averaged and local Nusselt number, ratios of secondary flow and drag coefficient. It is observed that the heat transfer at the bottom wall is substantially dependent on the rotational model and rotational speed of the cylinder.     

A numerical study of convective heat transfer from a rotating cylinder with cross-flow oscillation

This article presents a numerical investigation of convective heat transfer from a rotating cylinder with cross-flow oscillation. A finite element analysis using Characteristic Based Split method (CBS) is developed to solve governing equations involving continuity, Navier–Stokes, and energy equations. Dynamic unstructured triangular grid is used employing improved lineal and torsional spring analogy which is coupled with the solver by the Arbitrary Lagrangian–Eulerian (ALE) formulation. After verifying the numerical code accuracy, simulations are conducted to study convective heat transfer past a rotating cylinder with cross-flow oscillation at Reynolds numbers of 50, 100, and 200. Different rotational speeds of the cylinder normalized by free stream velocity, in the range of 0–2.5 are considered at various oscillating amplitudes and frequencies and three different Prandtl numbers of 0.7, 6, and 20. Effects of oscillation and rotation of cylinder on the temperature and flow field, vortex lock-on, mean Nusselt number, and the pattern of vortex shedding are investigated in detail considering iso-temperature and iso-flux boundary conditions on the cylinder surface. It is found that similar to the fixed cylinder, beyond a critical rotating speed, vortex shedding is mainly suppressed. Also by increasing the non-dimensional rotational speed of the cylinder, both the Nusselt number and the drag coefficient decrease rapidly. However, in vortex lock-on region, the Nusselt number increases in a large amount.     

A novel analytical solution of the non-uniform convective boundary conditions problem for heat conduction in cylinders

The problem of heat conduction in a still cylinder exposed to non‐uniform convective conditions on both inner and outer surfaces has been addressed by a method based on the series solution and a novel analytic solution is derived to predict the temperature field in the cylinder. Compared to previous available results this method allows a simpler implementation and its almost straightforward extension to multilayered cylinders represents one of the main advantages over more complex numerical solutions. As an example of application the effect of the non-uniform distribution of heat transfer coefficients on the solid temperature field and the heat transferred is analysed, as function of the fluid flow regime, the Biot number and the cylinder thickness.