Heat Transfer In Separated Flow Research Articles

Numerical study of turbulent heat transfer in separated flow: review

The numerical studies of turbulent heat transfer in separation flow presented in this paper. Enhancement of heat transfer rate in turbulent separation flow at sudden expansion in passage, over forward or backward facing-steps, blunt body, ribs channel, and swirl generators in channels were investigated numerically. Different models (CFD) used to study heat transfer characteristics and fluid flow in separation and reattachment region and compared results with previous experimental data. The effect of expansion ratio, Reynolds number, step height, (shape, number, and angle) of ribs and (length, twist angle, and gap width) twist the tape on improvement of heat transfer were referred. The numerical results indicated increases of heat transfer coefficients with increases in the above parameters. The numerical simulations derived from finite volume, element, and difference methods for evaluation of turbulent heat transfer in separated flow and employed several computational programs.

An analysis of flow and heat transfer in separated flows over blocked surface

The flow and heat transfer characteristics of flat and blocked surfaces were experimentally examined under the influence of the free stream velocity of 3, 5, 10 and 15 m/s encompassing laminar, transitional and turbulent flows. A constant-temperature hot wire anemometer was used for the velocity and turbulent intensity measurements, and copper-constant thermocouples and a micro-manometer for temperature and static pressures measurements, respectively. The flow over blocked surface separated in front of the first block and attached on it, then circulated between blocks, and then reattached behind the last block. The results showed that the flow separation before the first block occurred earlier in laminar–laminar separated–reattached flow than the transitional and turbulent flows and turbulent–turbulent separated–reattached flow leading to a shorter reattachment region with high free-stream turbulence. The presence of the separation and reattachment caused the heat transfer enhancement, which was more pronounced in the laminar flow and new empirical equations were developed for the local Stanton numbers.

Heat Transfer in Separated Flows on the Pressure Side of Turbine Blades

Heat transfer in separated flows on the pressure side of a typical high lift turbine profile is numerically investigated by means of an in-house CFD code. The numerical code was first validated on attached flows in turbine blades. To obtain flow separation cases, the profile is subject to large negative incidences so that a separation bubble is obtained at the pressure side. The numerical results are compared to available experimental data for code validation. It is shown how local minima and maxima values of the heat transfer coefficient are related to the separation and reattachment points, where the velocity component perpendicular to the wall is shown to have a significant effect on the heat transfer.

Predictions of turbulent flow and heat transfer in gas–droplets flow downstream of a sudden pipe expansion

Droplets-laden turbulent flow downstream of a sudden pipe expansion has been investigated by using Euler/Euler two-fluid model for the gaseous and dispersed phases. Significant increase of heat transfer in separated flow at the adding of evaporating droplets has been demonstrated (more than 2 times compare with one-phase flow at the value of mass concentration of droplets M L 1 ⩽ 0.05). Addition of dispersed phase to the turbulent gas flow results in insignificant increase of the reattachment length. Low-inertia droplets ( d 1 ⩽ 50 μm) are well entrained into the circulation flow and present over the whole pipe section. Large particles ( d 1 ≈ 100 μm) go through the shear layer not getting into the detached area. Comparison with experimental data on separated gas–droplets flows behind the plane backward-facing step has been carried out.

Heat Transfer in Separated Flow behind a Circular Cylinder for Reynolds Numbers from 120 to 30000. 1st Report. Time-Averaged Characteristics.

An experimental study was performed to investigate the heat transfer in separated flow behind a circular cylinder. The Reynolds number ranged from 120 to 30000. The temperature distribution around the cylinder was measured by an infrared camera under the condition of constant heat flux. From the results of the present study, it was found that the heat transfer in the separated flow region is strongly affected by the vortex formation length behind the cylinder. The value of heat transfer coefficient at the rear stagnation, Nur/Re0.5, has a maximum at Re≈200, and has a minimum at Re=1500-3000, corresponding to the change of the vortex formation length.

Augmented forced convection heat transfer in separated flow around a blunt flat plate

Results of numerical and experimental investigations of large-scale structures, both vortical and thermal, in the separating and reattaching flow on a heated rectangular plate are presented. A two-dimensional acoustic field is applied to lock the rate of vortex shedding from the leading-edge separation bubble and set the mean reattachment length. From autocorrelations and spectral analyses of the forced flow, it is found that large-scale vortex structures form in the separating shear layer at the acoustic forcing frequency and shed into the turbulent boundary layer. Downstream of reattachment, autocorrelations of the velocity fluctuations at a point near the plate show the dominant frequency of the velocity fluctuations to be half the applied acoustic frequency. The level of the applied acoustic field is set to optimize this correlation and to give a frequency comparable to the dominant frequency of vortex shedding in the natural (unforced) shedding case. A qualitative understanding of the instantaneous flow structures and the thermal structures forming on a heated plate is given by visualization using hydrogen bubbles in a water tunnel and Schlieren photography, respectively. A two-dimensional numerical model is used to predict the instantaneous flow structures and thermal field; good qualitative agreement is found between the predictions and the observations. Interpretation is made of the increased heat transfer rate on the plate surface as a result of the action of the large-scale vortex structures.

Negative heat transfer in separated flows

Negative heat transfer in separated flows

Effect of magnetic fluid coating on heat transfer in separated flow over a cylinder

In [i] the problem of isothermal flow over a cylindrical conductor covered with a thin layer of magnetic fluid was solved. Reynolds number interval 0.i ~ Re ~ i00 was considered and it was shown that the application of a magnetic fluid coating to the surface of the cylinder can considerably reduce the hydrodynamic drag. Another important characteristic of the flow over a body is the heat transfer rate. It is very tempting to test the possibility of using a magnetic fluid coating as a means of heat transfer control. To this end we will consider the problem described in [I] on the assumption that the temperature of the cylinder is higher than that of the free stream.

Application of a Viscous-Inviscid Interaction Procedure to Predict Separated Flows With Heat Transfer

A viscous-inviscid interaction procedure is described for predicting heat transfer in separated flows. The separating flow in a rearward-facing step/asymmetric channel expansion is considered. For viscous regions, the boundary layer momentum and continuity equations are solved inversely in a coupled manner by a finite-difference numerical scheme. The streamwise convective term is altered to permit marching the solution through regions of reversed flow. The inviscid flow is computed by numerically solving the Laplace equation for streamfunction in the region bounded by the displacement surfaces used in the inverse boundary layer solution. The viscous and inviscid solutions are repeated iteratively until the edge velocities obtained from both solutions are in agreement. Predictions using this method compare favorably with experimental data and other predictions.

Free-stream turbulence effects on heat transfer in the separated region of a circular cylinder. Effects of low frequency turbulence.

The effects of low frequency components of free-stream turbulence on heat transfer in the separated region of a cylinder were experimentally investigated for a Reynolds number range from 7.0×103 to 2.4×104 at the free-stream turbulence intensities of Tu < 0.5%. The experiments were carried out under constant heat flux conditions and two kinds of low frequency components of free-stream turbulence. The shape of separated shear layer, power spectrum of fluctuating velocity in the shear layer and pressure distribution around a cylinder were also measured. It is found that heat transfer in separated flow depends upon the diffusion of the separated shear layer. When the separated shear layer is thick and the effect of diffusion is large (Pattern Ap), heat transfer rate increases and vice versa ( Pattern Bp). Transition from Pattern Bp to Pattern Ap depends upon low frequency components of free-stream turbulence.

Heat transfer in turbulent flow with separation

The Reynolds analogy, which states that the wall friction is proportional to the heat flux, is not valid for flow separation regions, since then 3u/~y -O, while in some cases the heat flux reaches values even greater than in reattached flow. It is expedient to use the turbulent energy balance equation to calculate heat transfer in separated flows. In spite of the fact that in separated flow the friction stress at the wall is zero, the turbulent energy intensity in the flow is large; the turbulence diffuses towards the wall and, in particular, accounts for the intense heat transfer. The one-dimensional turbulent energy balance equation for separated flow was considered in [i], where the authors analyzed the limiting case of flow separation where there is no turbulent energy production (rdu/dy = 0) throughout the entire flow. It is interesting to consider the case of a linear distribution of friction stress across the flow, with zero stress at the wall. This distribution, as was pointed out in [2], agrees with experimental data for separated flow.

Steady Laminar Forced Convection from a Circular Cylinder at Low Reynolds Numbers

Comparison of experimental data for the heat transfer between a circular cylinder and a steady stream of viscous, incompressible fluid at low Reynolds numbers with the only available theoretical solution, that based on Oseen type linearization of the heat transfer equation, indicates considerable discrepancies. This question can only be resolved by obtaining solutions of the heat transfer equation based on the correct velocity distribution in the field of flow according to the full Navier-Stokes equations. This problem is considered for a range of Reynolds numbers from 0.01 up to 40. Solutions of the equation for forced heat convection are obtained using numerical methods. The mean and local Nusselt numbers are calculated and compared with available experimental data and correlations. The results are found to compare favorably. At the upper end of the Reynolds number range the solutions are examined to see if their trend checks with boundary-layer results at high Reynolds numbers and with some recent predictions as to the nature of the heat transfer in separated flow past a circular cylinder at high Reynolds numbers. Again some evidence of satisfactory comparison is found.

The influence of sound on heat transfer in separated flows

The flow in the transition region of the wake behind a circular cylinder transverse to the flow is discussed in the light of recent detailed hot-wire studies, and it is found that these studies present a completely consistent qualitative explanation of the heat transfer measurements of FAND and CHENG in the range of crossflow Reynolds numbers from 4000 to 11,000.