Kahoru Torii

Heat transfer enhancement accompanying pressure-loss reduction with winglet-type vortex generators for fin-tube heat exchangers

Abstract This paper proposes a novel technique that can augment heat transfer but nevertheless can reduce pressure-loss in a fin-tube heat exchanger with circular tubes in a relatively low Reynolds number flow, by deploying delta winglet-type vortex generators. The winglets are placed with a heretofore-unused orientation for the purpose of augmentation of heat transfer. This orientation is called as “common flow up” configuration. The proposed configuration causes significant separation delay, reduces form drag, and removes the zone of poor heat transfer from the near-wake of the tubes. This enhancement strategy has been successfully verified by experiments in the proposed configuration. In case of staggered tube banks, the heat transfer was augmented by 30% to 10%, and yet the pressure loss was reduced by 55% to 34% for the Reynolds number (based on two times channel height) ranging from 350 to 2100, when the present winglets were added. In case of in-line tube banks, these were found to be 20% to 10% augmentation, and 15% to 8% reduction, respectively.

Numerical and experimental determination of flow structure and heat transfer effects of longitudinal vortices in a channel flow

Longitudinal vortices have enormous utility for flow control. Longitudinal vortices are also capable of producing beneficial effects in transport enhancement. The vortices disrupt the growth of the boundary layer and serve ultimately to bring about enhancement of heat transfer between the fluid and its neighboring surface. The present study determines the flow structure, in detail, behind a winglet type vortex generator placed in a fully developed laminar channel flow. The flow structure is complex and consists of a main vortex, a corner vortex and induced vortices. Experiments are performed in order to corroborate the numerical predictions of the flow structure. The purpose of this study is to show the performance of a delta winglet type vortex generator in improving heat transfer. The conclusions that are drawn identify plausible choice regarding the optimal angle of attack of the vortex generator. Such vortex generators show great promise for enhancing the heat transfer rate in plate-fin crossflow heat exchangers.

Turbulence statistics in the stagnation region of an axisymmetric impinging jet flow

Turbulence statistics in the stagnation region of an axisymmetric jet impinging vertically on a flat plate are reported. The measurements were made in a submerged water jet facility at a Reynolds number of approximately 13,000 based on the nozzle exit velocity and the nozzle diameter. Particle-tracking velocimetry was used for the measurement of highly turbulent flows near the stagnation point. The axial mean momentum balance was examined to clarify the effect of the turbulent normal stress on the axial mean momentum transport. It was found that the turbulent normal stress of the axial component made a substantial contribution to the increase in the static pressure near the impingement wall. Detailed distributions of the turbulent stresses and the triple correlations of velocity fluctuations are presented. The state of turbulence was studied by means of an invariant map of the turbulent stress anisotropy. It was revealed that the turbulence was close to an axisymmetric state in the stagnation region. The budget equation for the turbulent kinetic energy evaluated from the present data shows that the net negative production takes place in the vicinity of the wall. This negative production is compensated by the pressure diffusion. Other unique features of the budget for the turbulent kinetic energy are also discussed.

Statistical simulation of particle deposition on the wall from turbulent dispersed pipe flow

Deposition of particles towards the wall from a turbulent dispersed flow in a vertical pipe has been studied numerically. A fully developed turbulent pipe flow of air is chosen as the primary flow, and it is represented by the law-of-the-wall relations and the average turbulence statistics obtained from a direct numerical simulation reported in the literature. Trajectories and velocities of the particles are calculated, using a one-way coupling Lagrangian eddy–particle interaction model. Thousands of individual particles (typically 920 kg/m3 in density) of various diameters (2.0–68.5 μm) are released in the represented flow, and deposition velocities are evaluated. It is shown that the deposition velocities predicted are in good agreement with experimental data available in the literature. The influence of some forces in the particle equation of motion (i.e., the Saffman lift force, the centrifugal force, the conservation of angular momentum and the buoyancy force) on the prediction of the deposition velocities is examined. Also examined is the influence of the inlet particle concentration profile, on which little attention has been paid so far. The unique phenomenon of ‘near-wall build-up’ of small particles, which has been reported in some previous simulations and experiments, was also observed in the present simulation while the result for very small particles (τp+<3) should be accepted with reservation due to their possible spurious build-up associated with the random-walk approach.

Stereo imaging for simultaneous measurement of size and velocity of particles in dispersed two-phase flow

A stereo-imaging technique for the simultaneous measurement of size and velocity of solid/liquid particles in dispersed two-phase flows is developed. Particles are illuminated with back light provided by the two strobe lamps. Double-pulsed strobe flashing is done synchronously with the framing of the CCD camera to achieve the frame-straddling illumination mode for the measurement of particle velocities. Silhouetted particle images are acquired with two black-and-white CCD cameras in stereo configuration. The particle images are analysed with a specially devised procedure, which can faithfully detect the perimeters both of spherical and of non-spherical particle images. The use of stereo imaging permits measurement of all three velocity components and also resolution of the depth-of-field effect in particle sizing, the problem that has been a key subject in the development of accurate particle-sizing techniques based on back lighting. The technique developed here is capable of sizing 10-500µm particles to within ±4µm inaccuracy. The validity of the technique is demonstrated by the measurement of a variety of transparent/opaque and spherical/non-spherical particles falling down a vertical pipe.

Thermal contact conductance under low applied load in a vacuum environment

The thermal contact conductance in a vacuum environment under low applied load was studied with square test plates (100 X 100 mm) made of aluminum alloy (A6061 and A5052). Two kinds of contact geometries were examined: the contact between a practically flat rough surface and an approximately spherical one, and the contact of similarly flat rough surfaces. Heat transfer experiments were carried out in a vacuum (< 13.3 Pa) in a contact pressure range of 0.1–0.6 MPa. These experimental conditions simulated an actual heat transfer surface of a cold-plate heat exchanger that is scheduled to be used in outer space. By using a pressure-measuring film that is capable of visualizing contact pressure distributions, a new technique for predicting the thermal contact conductance was developed. The technique evaluates the microscopic and macroscopic thermal constriction resistances from the real contact pressure distribution, which is measured by means of digital image processing from the color density pattern appearing in the film. A procedure was also devised to reevaluate the real contact pressure distribution that would occur in direct metal-to-metal contact without the pressure-measuring film. It is shown that the values of thermal contact conductance predicted by the present technique are in excellent agreement with those obtained by the heat transfer experiment, thus demonstrating the usefulness of the present technique as a practical tool for predicting thermal contact conductance. In contrast, the theory and laboratory correlations reported in previous studies were not applicable under the conditions examined here owing to insufficient accuracy. The present results clearly indicate that the macroscopic constriction resistance caused by the surface waviness and/or deformation of the substrate is predominant under low applied loading, and, therefore, its evaluation is crucial to the better design of efficient heat exchangers for use in spacecraft applications.

Heat transfer and pressure loss penalty for the number of tube rows of staggered finned-tube bundles with a single transverse row of winglets

Abstract The objective of this research is to investigate the heat transfer and pressure loss penalty for various numbers of transverse rows in staggered finned-tube bundles with a single transverse row of the winglet pairs beside the front row of the tube bundles. Experiments were performed for two, three, four and five rows of staggered tube bundles. The pairs of winglets were placed with a heretofore-unused orientation for the purpose of augmentation of heat transfer and reduction of pressure loss penalty. This orientation is called as “common flow up” configuration. For three rows of tubes with a single transverse row of winglets beside the front row of the tubes, the heat transfer was augmented by 30–10%, and yet the pressure loss was reduced by 55–34% with the increase of the Reynolds number (based on two times channel height) from 350 to 2100. The reduction of the pressure loss penalty for three rows of tube bundles is the largest in comparison with the other numbers of rows.

HEAT TRANSFER AUGMENTATION BY LONGITUDINAL VORTICES ROWS

Longitudinal vortices can potentially enhance heat transfer with small pressure loss penalty. The objective of this experimental work is to investigate the influence of arrays of longitudinal vortices generated by half-delta wings on the local and average heat transfer and fluid flow of an otherwise laminar boundary layer. Heat transfer experiments and hot-wire velocity measurements were conducted in order to clarify the mechanisms of heat transfer augmentation. Experiments with arrays of co-rotating and counter-rotating longitudinal vortices indicated that both laminar and turbulent effects play a key role in enhancing heat transfer. Counter-rotating longitudinal vortices tend to merge in the common flow up region between generators; in this case, smaller distances between generators with the common flow down and large angles of attack were found to enhance the heat transfer. Arrays of co-rotating longitudinal vortices show lower vortex merging; arrays of vortices produced by half-delta wings with higher angles of attack significantly distort the flow field and produce a noticeable growth of near-wall turbulence intensity. An overall performance analysis indicated that arrays of counter-rotating longitudinal vortices are more suitable for heat transfer enhancement than arrays of co-rotating vortices.

ENHANCEMENT OF LAMINAR BOUNDARY LAYER HEAT TRANSFER BY LONGITUDINAL VORTICES

This chapter focuses on the enhancement of laminar boundary layer heat transfer by longitudinal vortices with a small additional pressure loss. Longitudinal vortices potentially enhance the heat transfer with a small additional pressure loss. Longitudinal vortices generated by half-delta wings attached to the surface at an angle of attack persist over a long streamwise distance, disturbing the entire velocity and temperature fields. The chapter also discusses a series of experimental and numerical works conducted aiming to understand the heat transfer aspect of flows dominated by longitudinal vortices, and to clarify the mechanism of heat transfer augmentation and get insights in order to propose vortex generators configuration for use in heat transfer equipment. Considerable enhancement of heat transfer occurs with a modest pressure loss penalty. The chapter further discusses the effects of longitudinal vortices generated by a single vortex generator and a pair of vortex generators on heat transfer of an otherwise laminar boundary layer. It describes an experiment in which the angle of attack, height, and geometry of the generators served as a parameter for heat transfer measurements closer to the vortex generators. It was found that large local enhancement was achieved in regions where the regime is predominantly laminar by the boundary layer thinning due to the vortical motion.

Steady state comparative-longitudinal heat flow method using specimen of different thicknesses for measuring thermal conductivity of lotus-type porous metals

Lotus-type porous metal with many straight pores is attractive as a heat sink, because larger heat transfer capacity is obtained, owing to the small diameter of the pores. In order to use lotus-type porous metal effectively as a heat sink, it is important to clarify the effective conductivity and to consider the pore effect on heat conduction in the lotus-type porous metal. Because lotus-type porous metal is an anisotropic material, a steady state comparative-longitudinal heat flow (SCHF) method for thermal conductivity, referring to the ASTM standard, is thought to be better than a nonsteady method such as a laser flash method. This paper investigated the variable of the steady state comparative-longitudinal heat flow method by using specimens of different thicknesses (SCHF-DT) method for measuring the effective thermal conductivity. As a result, the uncertainty of effective thermal conductivity of a specimen was found to be smaller, as the diameter of a rod became larger and the length of a rod became shorter. In addition, it was found by error analysis that a dominant factor in the uncertainty of this method was the contact thermal difference between the rod and specimen.