N. K. Mitra

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.

Experimental investigations of heat transfer enhancement and flow losses in a channel with double rows of longitudinal vortex generators

Abstract Flow structure, heat transfer, and drag by longitudinal vortices generated by double rows of delta winglets in transition channel flow are investigated for the reduction of the gas side heat transfer resistance of compact heat exchangers. The experiments consist of flow visualization by laser light sheets, liquid crystal thermography for local heat transfer and balance measurements for drag. Angle of attack and channel Reynolds number have been varied. Aligned delta winglet double rows show higher heat transfer enhancement than staggered. The critical angle of attack for the formation of longitudinal vortices is smaller behind the second row than behind the first. Heat transfer enhancements of 80% and drag increases of 160% have been found on wall areas 40 times the winglet area. The ratio of heat transfer enhancement and drag increase is larger for higher Reynolds numbers.

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.

Comparison of Wing-Type Vortex Generators for Heat Transfer Enhancement in Channel Flows

Longitudinal vortices can be generated in a channel flow by punching or mounting small triangular or rectangular pieces on the channel wall. Depending on their forms, these vortex generators (VG) are called delta wing, rectangular wing, pair of delta winglets, and pair of rectangular winglets. The heat transfer enhancement and the flow losses incurred by these four basic forms of VGs have been measured and compared in the Reynolds number range of 2000 to 9000 and for angles of attack between 30 and 90 deg. Local heat transfer coefficients on the wall have been measured by liquid crystal thermography. Results show that winglets perform better than wings and a pair of delta winglets can enhance heat transfer by 46 percent at Re=2000 to 120 percent at Re=8000 over the heat transfer on a plate.

Heat transfer enhancement of finned oval tubes with staggered punched longitudinal vortex generators

Abstract Punched longitudinal vortex generators in form of winglets in staggered arrangements were employed to enhance heat transfers in high performance finned oval tube heat exchanger elements. Three-dimensional hydrodynamically and thermally developing laminar flow ( Re =300) and conjugate heat transfer in finned oval tubes were calculated by solving the Navier–Stokes and energy equations with a finite-volume method in curvilinear grids. Velocity field, pressure distribution, vortex formation, temperature fields, local heat transfer distributions and global results for finned oval tubes with two to four staggered winglets ( β =30°, Λ=2, h=H ) were presented and compared. Winglets in staggered arrangement bring larger heat transfer enhancement than in in-line arrangement since the longitudinal vortices from the former arrangement influence a larger area and intensify the fluid motion normal to the flow direction. For Re =300 and Fi =500, the ratios of heat transfer enhancement to flow loss penalty ( j/j 0 )/( f/f 0 ) were 1.151 and 1.097 for a finned oval tube with two and four staggered winglets, respectively.

Large-eddy simulation of flow and heat transfer in an impinging slot jet

The flow field due to an impinging jet at a moderately high Reynolds number, emanating from a rectangular slot nozzle has been computed using a large eddy simulation (LES) technique. A dynamic subgrid-scale stress model has been used for the small scales of turbulence. Quite a few successful applications of the dynamic subgrid-scale stress model use planar averaging to avoid ill conditioning of the model coefficient. However, a novel localization procedure has been attempted herein to find out the spatially varying model coefficient of the flow. The flow field is characterized by entrainment at the boundaries. Periodic boundary conditions could not be used on all the boundaries. The results reveal the nuances of the vortical structures that are characteristic of jet flows. The stress budget also captures a locally negative turbulence production rate. The calibration of the model has been made through prediction of the normalized axial velocity profile and heat transfer on the impingement plate. The computed results compare favorably with the experimental observations, especially in the stagnation zone.

Heat transfer enhancement of a finned oval tube with punched longitudinal vortex generators in-line

Abstract To explore the interaction of the vortical flow generated by punched delta-winglet pairs (DWPs) with in-line arrangement and to explore their influence on the heat transfer enhancement (HTE) and on flow loss penalty (FLP) in a high performance finned oval tube (FOT) heat exchanger element, three-dimensional flow and conjugate heat transfer in an FOT were calculated for a thermally and hydrodynamically developing laminar flow ( Re = 300) by solving the Navier–Stokes and energy equations with a Finite-Volume Method in body-fitted grids. The conjugate heat transfer was realized by iterations of the energy equation in the flow field and the conduction equation in the fin. FOT with one to three in-line DWPs ( β = 30°, Λ = 2, h = H ) were investigated. Velocity and temperature fields, vortex formation, local heat transfer distributions and global results were presented. The LVs of the incoming flow intensified the LVs downstream of the second and the third winglet. For Re = 300 and Fi = 500, the ratios of HTE to FLP ( j ⧹ j 0 ) ⧹ ( f ⧹ f 0 ) were 1.04, 1.01 and 0.97 for an FOT with one, two and three DWPs in-line respectively.

Conjugate heat transfer of a finned oval tube with a punched longitudinal vortex generator in form of a delta winglet—parametric investigations of the winglet

Abstract To explore the influences of the angle of attack and the aspect ratio of a winglet, which is punched near the leading edge of the fin in a high performance finned oval tube (FOT) , on the heat transfer enhancement (HTE) and flow loss penalty (FLP) , three-dimensional flow and conjugate heat transfer in a FOT were calculated for a thermally and hydrodynamically developing laminar flow (Re = 300) by solving the Navier–Stokes and energy equations with a Finite-Volume Method in body-fitted grids. The conjugate heat transfer was realized by iterations of the energy equation in the flow field and of the conduction equation in the fin. Three angles of attack (β = 20°, 30° and 45°) and two aspect ratios (Λ = 1.5 and 2) were investigated. Velocity and temperature fields, vortex formation, local heat transfer distributions and global results are presented. The winglet with β = 30° and Λ = 2 provides the best ratio of HTE to FLP with ( j\j0) \ ( f\f0) = 1.04.

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.