Martin Fiebig

Embedded vortices in internal flow: heat transfer and pressure loss enhancement

Abstract Vortices depend strongly on the way they are generated. A class of wing-type vortex generators, which can easily be incorporated into heat transfer surfaces, is considered in fully developed and developing channel flows with respect to heat transfer enhancement and pressure loss penalty. Such configurations have been investigated in detail by the research group “Vortices and Heat Transfer” at the ITF of the Ruhr University. Three enhancement mechanisms are identified and discussed quantitatively: (1) developing boundary layers on the vortex generator surface; (2) swirl; and (3) flow destabilization. Experiments and calculations show that (1) winglets are more effective than wings, and (2) rectangular and delta winglets give similar performance. A base configuration with rectangular vortex generators is investigated in detail. Transition starts at Reynolds numbers below 300, and no dominant frequencies can be detected at Re= 1,000. At all Reynolds numbers, longitudinal vortices are more effective than transverse vortices; i.e., longitudinal vortex generators generate higher heat transfer enhancement for the same pressure penalty than transverse vortex generators.

Heat transfer enhancement and drag by longitudinal vortex generators in channel flow

Abstract Triangular and rectangular longitudinal vortex generators were formed by punching small pieces out of flat plate fins so that they stuck out of the plates and formed an angle (angle of attack) with the main flow direction. The fins were mounted on top of each other to form channels, each representing an idealization of a gas-side element of a compact heat exchanger. The effects of single-vortex generators on flow structure, flow losses, and heat transfer were investigated. The vortex structure was observed, the drag—a measure of flow losses—was measured by a balance, and the local heat transfer coefficient was obtained from unsteadt liquid crystal thermography. Vortex generator geometry, angle of attack, and Reynolds number were varied. Stable longitudinal vortices were found up to much higher angles of attack than for corresponding wings in a free stream. The drag induced by the vortex generators was found to be proportional to the projected area and independent of the shape and the Reynolds number. Local heat transfer augmentation of several hundred percent and mean heat transfer enhancement of more than 50% over an area more than 50 times the vortex generator area were achieved. The heat transfer enhancement per unit vortex generator area was highest for delta wings followed by delta winglets and rectangular winglets.

Heat transfer enhancement in fin-tube heat exchangers by winglet type vortex generators

Numerical investigations of the flow structure and heat transfer enhancement in a channel with a built-in circular tube and a winglet type vortex generator are presented. The geometrical configuration represents an element of a gas-liquid fin-tube crossflow heat exchanger. In the absence of the winglet type vortex generator, relatively little heat transfer takes place in the downstream of the circular tube which is a recirculation region with low velocity fluid. However, in the presence of a winglet type longitudinal vortex generator in the wake region behind the cylinder, heat transfer in this region can be enhanced as high as 240%. Results show a marked increase in overall channel heat transfer. The enhancement shows great promise in reducing the size of the heat exchangers.

Wing-type vortex generators for fin-and-tube heat exchangers

Abstract The effect of wing-type vortex generators on heat transfer and pressure drop of a fin-and-tube heat exchanger element was investigated. Local heat transfer was measured by liquid crystal thermography on the fin in the Reynolds number range of 600–2700. Flow losses were estimated from the measured pressure drop of an element. Delta winglets were used as vortex generators. Four fin-and-tube configurations were tested, an inline and a staggered arrangement, each with plain fins and with fins with a pair of vortex generators behind each tube. For the inline tube arrangement the vortex generators increase the heat transfer by 55–65% with a corresponding increase of 20–45% in the apparent friction factor. Results indicate that the vortex generators have the potential to reduce considerably the size and mass of heat exchangers for a given heat load.

Flow structure and heat transfer in a channel with multiple longitudinal vortex generators

Abstract Experimental results of flow structure and heat transfer enhancement by longitudinal vortices in plane channels built by parallel plates in the transition flow regime are presented. The longitudinal vortices are generated by single and double rows of delta half-wing vortex generators punched out of the channel wall. The investigations are performed by a laser light sheet technique for flow visualization and unsteady liquid-crystal thermography for local heat transfer measurements. For an aligned arrangement of two rows of vortex generators, the experiments show that the flow structure in the wake of the second row is qualitatively similar to that of the first row. The peak value of the span-averaged Nusselt number at the wake of the second row is strongly dependent on the spacing of the two rows. For steamwise distances between the two rows of about seven channel heights this peak value is larger than the peak value of the span-averaged Nusselt number at the wake of the first row.

Local heat transfer and flow losses in fin-and-tube heat exchangers with vortex generators: A comparison of round and flat tubes

Abstract Local heat transer on the plate fin of a fin-and-tube heat exchanger element with flat tubes in staggerred arrangement was measured by liquid crystal thermography in the Reynolds number range of 600–3000. The flow loss was estimated from the measure pressure loss. The influence of longitudinal vortex generators on fin heat transfer and floww losses was investigated. The results were compared with similar experimental results for round tubes. For the straggered fin-and-tube arrangement, the longitudinal vorices increase heat transfer only marginally (10%) for round tubes but dramatically (100%) for flat tubes. The heat exchanger element with flat tubes and vortex generators gives nearly twice as much heat transfer and only half as much pressure loss as the corresponding heat exchanger element with round tubes.

NUMERICAL INVESTIGATION OF MIXED CONVECTION HEAT TRANSFER IN A HORIZONTAL CHANNEL WITH A BUILT-IN SQUARE CYLINDER

Structures of laminar wakes and heat transfer in the presence of thermal buoyancy art investigated from the numerical solution of complete Navier-Stokes and energy equations in a two-dimensional horizontal channel with a built-in square cylinder. Results show that mixed convection can initiate periodicity and asymmetry in the wake at lower Reynolds numbers than forced convection alone. For a given Reynolds number, the heating of the fluid in the channel is improved by mixed convection up to a certain Grashof number and deteriorates if the Grashof number is further increased.

CONJUGATE HEAT TRANSFER OF A FINNED TUBE PART A: HEAT TRANSFER BEHAVIOR AND OCCURRENCE OF HEAT TRANSFER REVERSAL

The conjugate heat transfer in a high-performance finned-lube heat exchanger element was calculated for three-dimensional thermally and hydrodynamically developing laminar flows. The influence of Reynolds number Re and a fin efficiency parameter Fi (ratio of fin to fluid conductivity times fin thickness to fin pitch) on the heat transfer behavior has been studied. Flow patterns, pressure distribution. Nusselt number distribution, heat flux distribution, and fin efficiency are presented. The part of the fin upstream of the tube is much more efficient than the downstream part. A unique heat transfer phenomenon, a directional reversal of the heat transfer, occurred locally on the fin in the tube wake for small Fi and large Re. This can be interpreted as three-dimensional interaction of convection and the fin conduction in the tube wake, when the flow is dominated by a strong horseshoe vortex and a dead water zone with recirculation.

STRUCTURE OF VELOCITY AND TEMPERATURE FIELDS IN LAMINAR CHANNEL FLOWS WITH LONGITUDINAL VORTEX GENERATORS

Laminar velocity and temperature fields in a rectangular channel with a row of built-in vortex generators in the form of slender delta wings and winglet pairs have been calculated by means of a ifinal method consisting of zones of complete and partially parabolized Navier-Stokes and energy equations. A modified version of SOLA for incompressible as well as for variable-density, small-Mach number flows has been used to solve the basic equations. Each wing or winglet pair generates counterrotating longitudinal vortices that, in contrast to similar vortices in an unbounded medium, show an elliptic deformation, a wakelike axial velocity distribution in the core, and an absence of breakdown even at angles of attack as large as 50° The spiraling motion induced by these vortices in the channel can locally enhance the heat transfer coefficient by a factor of 3 compared to its value in a wingless channel. The temperature dependence of density can reduce this enchancement by roughly 10% when the ratio of gas-to-wal...

Color-based image processing to measure local temperature distributions by wide-band liquid crystal thermography

This study presents a color-image-processing procedure for non-intrusive local temperature measurements by thermochromic liquid crystals (TLCs). The image evaluation software is completely independent of the color detection and acquisition hardware. This allows to use a wide variety of hardware solutions. An easy reproducible calibration of camera and light source is presented. The dependence of the detected hue values on intensity is investigated and further the hueversus temperature relation is studied.