M. Fiebig

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.

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.

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.

Numerical investigation of turbulent flows and heat transfer in a rib-roughened channel with longitudinal vortex generators

Abstract Three-dimensional turbulent flows and heat transfer in a rectangular channel with longitudinal vortex generators on one wall and rib-roughness elements on the other wall have been modeled by the κ-e model and law of the wall and computed. Rectangular winglets have been used as vortex generators. Results show that the combined effect of rib-roughness and vortex generators can enhance the average Nusselt number by nearly 450%.

Performance evaluation of a vortex generator heat transfer surface and comparison with different high performance surfaces

Abstract A comparative assessment of five different heat transfer configurations for operation in compact heat exchangers is presented. The configurations under consideration are four standard heat exchanger surfaces—two plain fin, an offset strip and a louvered fin geometry—and one surface with so called vortex generators for heat transfer augmentation. In the case of the standard surfaces, the basic performance characteristics in the form of heat transfer and friction data versus the Reynolds number have been taken from published experimental results. In the case of the vortex generator surface, the performance characteristics have been derived from a numerical prediction of the flow and temperature field in a closely spaced parallel plate channel with vortex generators in the form of delta wings mounted on the channel walls. In comparison to the plain fin surface with a rectangular cross section, the vortex generator surface shows best performance characteristics allowing a reduction in heat transfer surface area of 76%, for fixed heat duty and for fixed pumping power.

Effects of longitudinal vortex generators on heat transfer and flow loss in turbulent channel flows

Abstract The influences of four types of longitudinal vortex generators (delta wing, rectangular wing, delta winglet pair and rectangular winglet pair) on heat transfer and flow loss in turbulent channel flows are investigated in the present work. A numerical solver of the Navier-Stokes equations, based on the MAC algorithm, has been extended with the k-ɛ . turbulence model for this study. The results show that the longitudinal vortices produced by the vortex generators elevate significantly the level of turbulence kinetic energy in the flows and strongly disturb the thermal boundary layer. The mean heat transfer rate can be increased by 16–19% with the vortex generators for an area of channel wall which is 30 times larger than the vortex generator area. The ratio of heat transfer enhancement and flow loss increase indicates better performance for the rectangular winglet pair.

Impinging radial and inline jets : A comparison with regard to heat transfer, wall pressure distribution, and pressure loss

Heat transfer and wall pressure distribution on a plane surface generated by single impinging inline or radial jets are studied experimentally. The pressure drop of inline and radial jet nozzles is measured. The effects of flow exit angle, nozzle to surface distance, and exit velocity on heat transfer, wall pressure distribution, and pressure drop are discussed. Heat transfer results show that radial jets with flow exit angles of +45°–+60° generate up to 60% higher local and up to 50% higher global Nusselt numbers compared with inline jets of the same volumetric flow rate and exit velocity. Measured wall pressure distributions are presented in terms of pressure coefficients. The total force exerted by radial jets on a plane surface is lower than that exerted by inline jets. Radial jets with negative flow exit angles can generate small lifting forces. Results of pressure drop measurements are presented in terms of resistance coefficients, which allow an estimation of the necessary additional fan power if radial jet nozzles instead of inline jet nozzles are employed. For radial jet nozzles with flow exit angles of +45° ≤ θ ≤ +60° the rise of fan energy costs is negligible compared with the rise of heat/mass transfer. Radial jet nozzles have a high potential for application, particularly when very high drying rates or small jet forces on the impingement surface or both are required.

Thermal diffusivity of transparent liquids by photon correlation spectroscopy—II. Measurements in binary mixtures with a small difference in the refractive index of both pure components

Abstract Photon correlation spectroscopy has been used successfully for the measurement of the thermal diffusivity of some selected binary liquid mixtures. Under the condition, that the difference of the refractive indices of both pure components is smaller than 5%, the thermal diffusivity can be determined independently on the diffusion coefficient, which in general also gives contributions to the measured signal. For room temperature and atmospheric pressure, the results for benzene-toluene, carbon tetrachloride-toluene, decalin-toluene, and bromobenzene-toluene systems are presented as functions of the weight fraction.