An experimental investigation of heat transfer and flow friction characteristics of louvered fin surfaces by the modified single blow technique

This paper describes the development of an experimental facility to determine the heat transfer and flow friction characteristics of heat exchange surfaces by the modified single blow technique and the application of this transient technique to evaluate the performance characteristics of louvered fin heat exchangers. The reliability of implementing the modified single blow technique on the developed test facility is borne out by the good agreement in the heat transfer and flow friction data for the parallel plate test core when compared with theoretical and empirical correlations available in the literature. Performance evaluation of two louvered fin surfaces used mainly for cooling of large land and marine based electrical power generator sets is carried out and compared with similar louvered fin surfaces available in the literature. On the basis of dimensionless area and power factors, it was found that the flat fin is slightly superior in overall performance than its corrugated counterpart for low Reynolds numbers. Both surfaces are however inferior in performance when compared with the flat fin surface of Achaichia and Cowell and the corrugated fin surface of Davenport. Use of the j/f ratio as an approximate figure of merit led to an inaccurate assessment of the performance of the louvered fin heat exchanger surfaces evaluated in this study. Received on 8 May 1998     

Entry region of louvered fin heat exchangers

The dominant thermal resistance for most compact heat exchangers occurs on the gas side and as such an understanding of the gas side flowfield is needed before improving current designs. Louvered fins are commonly used in many compact heat exchangers to increase the surface area and initiate new boundary layer growth. For this study, detailed flowfield measurements were made in the entry region of several louvered fin geometries whereby the louver angle, ratio of fin pitch to louver pitch, and Reynolds number were all varied. In addition to mean velocity measurements, time-resolved velocity measurements were made to quantify unsteady effects.

The results indicated larger fin pitches resulted in lower average flow angles in the louver passages and longer development lengths. Larger louver angles with a constant ratio of fin pitch to louver pitch resulted in higher average flow angles and shorter development lengths. As the Reynolds number increased, longer development lengths were required and higher average flow angles occurred as compared with a lower Reynolds number case. Time-resolved velocity measurements indicated some flow periodicity behind the fully developed louver for a range of Reynolds numbers. The Strouhal number of these fluctuations was constant for a given louver geometry, but the value increased with increasing fin pitch.     

Experimental study of convective heat transfer from a rotating finned tube in transverse air flow

 The convective heat transfer from fins to air has been evaluated using rotating annular fins subjected to an air flow parallel to the fins. The fin cooling is studied using infrared thermography. The thermal balance in a fin during its cooling process allows us to obtain the heat transfer coefficient from the temperature time evolution of the fin. Moreover, Particle Image Velocimetry allows us to obtain the flow field in the mid-plane between two fins. The influence of the fin spacing on the convective heat transfer is studied for various velocities of the superposed air flow and various fin rotational speeds. These tests were carried out for air flow Reynolds numbers (based on the shaft diameter and the velocity of the superposed air flow) between 2550 and 18200 and rotational Reynolds numbers (based on the shaft diameter and the peripheral speed) between 800 and 2.9 × 10⁴, for different fin spacings. Received: 14 May 1999/Accepted: 8 October 1999     

Unsteady mixed convection heat transfer from tandem square cylinders in cross flow at low Reynolds numbers

A two-dimensional numerical study is carried out to understand the influence of cross buoyancy on the vortex shedding processes behind two equal isothermal square cylinders placed in a tandem arrangement at low Reynolds numbers. The spacing between the cylinders is fixed with five widths of the cylinder dimension. The flow is considered in an unbounded medium, however, fictitious confining boundaries are chosen to make the problem computationally feasible. Numerical calculations are performed by using a finite volume method based on the PISO algorithm in a collocated grid system. The range of Reynolds number is chosen to be 50–150. The flow is unsteady laminar and two-dimensional in this Reynolds number range. The mixed convection effect is studied for Richardson number range of 0–2 and the Prandtl number is chosen constant as 0.71. The effect of superimposed thermal buoyancy on flow and isotherm patterns are presented and discussed. The global flow and heat transfer quantities such as overall drag and lift coefficients, local and surface average Nusselt numbers and Strouhal number are calculated and discussed for various Reynolds and Richardson numbers.     

Transition phenomena in the wake of an inclined square cylinder

The division of flow regimes in a square cylinder wake at various angles of attack (α) is studied. This study provides evidence of the existence of modes A and B instabilities in the wake of an inclined square cylinder. The critical Reynolds numbers for the inception of these instability modes were identified through the determination of discontinuities in the Strouhal number versus Reynolds number curves. The spectra and time traces of wake streamwise velocity were observed to display three distinct patterns in different flow regimes. Streamwise vortices with different wavelengths at various Reynolds numbers were visualized. A PIV technique was employed to quantitatively measure the parameters of wake vortices. The wavelengths of the streamwise vortices in the modes A and B regimes were measured by using the auto-correlation method. From the present investigation, the square cylinder wake at various angles of attack undergoes a similar transition path to that of a circular cylinder, although various quantitative parameters measured which include the critical Reynolds numbers, spanwise wavelength of secondary vortices, and the circulation and vorticity of wake vortices all show an α dependence.     

The effective installation of an inclined plate for the enhancement of forced convection over rectangular sources

This paper investigates the unsteady flow and heat transfer characteristics of rectangular sources with and without an inclined plate in a channel. The results show that the installation of an inclined plate above an upstream source results in a periodically unsteady flow and it can effectively enhance the heat transfer performance. Moreover, the inclined angle of the plate is changed under different Reynolds numbers to study the heat transfer performance in the channel. Received on 18 June 1997     

Flow and heat transfer in a pipe with a fin attached to inner wall

Enhancement of heat transfer to the fluid can be done by turbulence promoters such as attached fins to the pipe walls. In this study, the flow field and the heat transfer rates were numerically investigated in a pipe with an internally attached fin. Numerical simulations were conducted for four different types of fluids and for different fin heights and locations, and as the Reynolds number was varied, the effects of the fin on Nusselt number and friction factors were investigated. For all the Reynolds numbers considered in this study, the effect of fin location on the heat transfer rate and friction factor was negligible. As the fin height was increased, the mean Nusselt number and the friction factor also increased in the turbulent flow regimes. For low Prandtl number fluids (Pr = 0.011), the main heat transfer mode is conduction, and hence the mean Nusselt number slightly affected the flow rates.     

Vortex shedding and structural loading characteristics of finned cylinders

The flow development and structural loading characteristics of cylinders with equispaced circular fins were studied experimentally for a range of fin pitches with constant fin thickness and diameter. The experiments were performed for a range of Reynolds numbers, corresponding to the shear layer transition turbulent shedding regime. Time-resolved planar Particle Image Velocimetry and direct mean drag and fluctuating lift measurements are employed to relate spatio-temporal flow development to structural loading. The results show that wake development is dominated by vortex shedding for all the cases examined. However, the fin pitch ratio has a significant effect on vortex shedding characteristics. The addition of fins increases the characteristic spatial and temporal scales of the main spanwise vortices forming in the near wake. As the fin pitch is decreased to a critical value, the coalescence of boundary layers between the adjacent fins leads to a significant enlargement of the vortex formation region. A modified vortex shedding frequency scaling is proposed, based on the effective diameter, that incorporates a Reynolds number dependence associated with the lateral boundary layers developing on the fin surfaces. A detailed analysis is conducted to characterize the strength of the vortical structures forming in the near wake. The addition of the fins is shown to produce a stabilizing effect on the roll-up process, associated with a reduction in the generation of smaller scale, three-dimensional structures. The results demonstrate that the addition of fins leads to an increase in the mean drag, which is driven primarily by the associated increase in skin friction. The significant effect of the fin pitch ratio on the characteristics of the shed vortices as well as the size of the vortex formation region is shown to lead to substantial variations in the fluctuating loads.     

Two-dimensional unsteady laminar flow of a power law fluid across a square cylinder

The two-dimensional and unsteady free stream flow of power law fluids past a long square cylinder has been investigated numerically in the range of conditions 60≤Re≤160 and 0.5≤n≤2.0. Over this range of Reynolds numbers, the flow is periodic in time. A semi-explicit finite volume method has been used on a non-uniform collocated grid arrangement to solve the governing equations. The global quantities such as drag coefficients, Strouhal number and the detailed kinematic variables like stream function, vorticity and so on, have been obtained for the above range of conditions. While, over this range of Reynolds number, the flow is known to be periodic in time for Newtonian fluids, a pseudo-periodic flow regime displaying more than one dominant frequency in the lift is observed for shear-thinning fluids. This seems to occur at Reynolds numbers of 120 and 140 for n=0.5 and 0.6, respectively. Broadly speaking, the smaller the value of the power law index, lower is the Reynolds number of the onset of the pseudo-periodic regime. This work is concerned only with the fully periodic regime and, therefore, the range of Reynolds numbers studied varies with the value of the power law index. Not withstanding this aspect, in particular here, the effects of Reynolds number and of the power law index have been elucidated in the unsteady laminar flow regime. The leading edge separation in shear-thinning fluids produces an increase in drag values with the increasing Reynolds number, while shear-thickening fluid behaviour delays this separation and shows the lowering of the drag coefficient with the Reynolds number. Also, the preliminary results suggest the transition from the steady to unsteady flow conditions to occur at lower Reynolds numbers in shear-thinning fluids than that in Newtonian fluids.     

Viscous liquid film flow down an inclined corrugated surface. Calculation of the flow stability to arbitrary perturbations using an integral method

Viscous liquid film flow along an inclined corrugated (sinusoidal) surface has been studied. Calculations were performed using an integral model. The stability of nonlinear steady-state flows to arbitrary perturbations was examined using the Floquet theory. It has been shown that for each type of corrugation there is a critical Reynolds number for which unstable perturbations occur. It has been found that this value greatly depends on the physical properties of the liquid and geometric parameters of the flow. In particular, in the case of film flow down a smooth wall, the critical waveformation parameter depends only on the angle of inclination of the flow surface. The values of the corrugation parameters (amplitude and period) were obtained for which the film flow down a wavy wall is stable to arbitrary perturbations up to moderate Reynolds numbers. Such parameter values exist for all investigated angles of inclination of the flow surface.     

Numerical investigations on vortical structures of viscous film flows along periodic rectangular corrugations

Two-dimensional gravity-driven film flows along a substrate with rectangular corrugations are studied numerically by using Finite Volume Method. The volume of fluid (VOF) method is utilized to capture the evolution of free surfaces. The film flows down an inclined plate are simulated to validate the numerical implementation of the present study. Results obtained indicate that the phase shift between the surface wave and the wall corrugation increases as the Reynolds number. The parametric studies on the interesting resonant phenomenon indicate that the peak Reynolds numbers increase as the raise of the wall depth or the decline of the inclination angle. The dependence of the flow fields is analyzed on the Reynolds numbers and wall depth in details. It is found that the vortical structures in the steady flows, either produced by the interaction between capillary wrinkling and inertia, or by the rectangular geometry, are closely related to the remarkable deformation of the free surfaces. This conclusion is also confirmed by the transient flow development of two typical simulations, i.e., flows in capillary–inertial regime and in inertial regime.     

Numerical prediction of vortex shedding behind a square cylinder

It is common knowledge that flow around bluff bodies exhibits oscillatory behaviour. The aim of the present study is to compute the steady two-dimensional flow around a square cylinder at different Reynolds numbers and to determine the onset of unsteadiness through a linear stability analysis of the steady flow. Stability of the steady flow to small two-dimensional perturbations is analysed by computing the evolution of these perturbations. An analysis of various time-stepping techniques is carried out to select the most appropriate technique for predicting the growth of the perturbations and hence the stability of the flow. The critical Reynolds number is determined from the growth rate of the perturbations. Computations are then made for periodic unsteady flow at a Reynolds number above the critical value. The predicted Strouhal number agrees well with experimental data. Heat transfer from the cylinder is also studied for the unsteady laminar flow.     

Aerodynamic control of bridge cables through shape modification: A preliminary study

This paper examines the viability of modifying bridge cable shape and surface for the purpose of controlling wind-induced vibrations. To this end, an extensive wind-tunnel test campaign was carried out on various cable shapes about the critical Reynolds number region. Cable shapes were chosen to passively modify the flow in a particular manner. Tested shapes included those which have some form of waviness, faceting and shrouding. Section models were tested using a static inclined rig, allowing them to be installed at yawed cable-wind angles for both smooth and turbulent flow conditions. The aerodynamic damping of the tested cylinders is evaluated by applying both 1- and 2-dof quasi-steady aerodynamic instability models. This allows for the prediction of regions of aerodynamic instability, as a function of flow angle and Reynolds number. Whilst the plain, wavy and faceted cylinders are predicted to suffer from either dry inclined galloping, “drag crisis” or Den Hartog galloping, the shrouded cylinder is found to be stable for all angles of attack, albeit with an increase in drag at typical design wind velocities. Finally, turbulent flow is found to introduce an increased amount of aerodynamic damping mainly by providing a more constant lift force over tested Reynolds numbers.     

Early stages of an impulsively started unsteady laminar flow past tapered trapezoidal cylinders

Characteristics of the developing recirculation region behind a tapered trapezoidal cylinder and its interaction with the separating shear layer from the leading edges were studied numerically for an impulsively started laminar flow. An unsteady stream function–vorticity formulation was used. The Reynolds numbers considered range from 25 to 1000. Pressure contours, surface pressure coefficient, wake length and drag coefficient were studied through the streamline flow field. Main flow and subflow regimes were identified by an analysis of the evolution of the flow characteristics. It was found that typically, for a given trapezoidal cylinder, flow starts with no separation. As time advances, the symmetrical standing zone of recirculation develops aft of the trapezoidal cylinder. The rate of growth in width, length and structure of the aft end eddies depends on the Reynolds number. In time, separated flow from the leading edges of the trapezoidal cylinder also develops and forms growing separation bubbles on the upper and lower inclined surfaces of the trapezoidal cylinder. As time advances, the separation bubbles on the upper and lower inclined surfaces of the cylinder grow towards the downstream regions and eventually merge with the swelling symmetrical eddies aft of the cylinder. This merging of the flows creates a complex flow regime with a disturbed tertiary flow zone near the merging junction. Eventually, depending on the Reynolds number and the tapered angle of the trapezoidal cylinder, the flow develops into a specific category of symmetrical standing recirculatory flow with its own distinct characteristics. Comparisons with the available results of other investigators showed very good agreement. © 1998 John Wiley & Sons, Ltd.     

Vortex shedding and three-dimensional behaviour of flow past a cylinder confined in a channel

A numerical investigation of the flow past a circular cylinder centred in a two-dimensional channel of varying width is presented. For low Reynolds numbers, the flow is steady. For higher Reynolds numbers, vortices begin to shed periodically from the cylinder. In general, the Strouhal frequency of the shedding vortices increases with blockage ratio. In addition, a two-dimensional instability of the periodic vortex shedding is found, both empirically and by means of a Floquet stability analysis. The instability leads to a beating behaviour in the lift and drag coefficients of the cylinder, which occurs at a Reynolds number higher than the critical Reynolds number for the three-dimensional mode A-type instability, but lower than a Reynolds number for any mode B-type instability.     

Flow stability of a conducting liquid flowing down an inclined plane in the presence of a magnetic field

The stability of a steady flow of incompressible, conducting liquid down an inclined plane in the presence of longitudinal and transverse magnetic fields is studied. Solutions of the linearized magnetohydrodynamic equations with corresponding boundary conditions are found on the assumption that the Reynolds number R_g and the wave number are small. It is shown that the longitudinal magnetic field plays a stabilizing role. It is known [1] that the flow of a viscous liquid over a vertical wall is always unstable. In this article it is shown that the instability effect at small wave numbers may be eliminated if the longitudinal magnetic field satisfies the conditions found. The case when the Alfvén number and the wave number are small and the Reynolds number is finite is also examined.     

Mean flow characteristics of a turbulent plane jet with buoyancy induced curvature

An experimental investigation of heated vertical and inclined plane air jets discharged into quiescent surroundings is described. A unique feature of this data is that Pilot tube measurements were used to define the mean trajectory of the inclined jets so that subsequent hot-wire traverses could be made normal to the curved path. While the mean velocity and temperature profiles are self-similar for the range of exit conditions studied, other aspects of the mean jet development depend on the exit Reynolds and Froude numbers, or the discharge angle. It is noted that variations between this study and other published data suggest further measurements of this flow situation are needed, with particular attention to specific features of the jet apparatus and ambient surroundings, and to the exit Reynolds number. Presently with Dept. of Mechanical Engineering, University of Alexandria, Alexandria, Egypt     

Unsteady Laminar to Turbulent Flow in a Spacer-Filled Channel

A combined numerical and experimental investigation has been carried out to study the flow behaviour in a spacer-filled channel, representative of those used in spiral-wound membrane modules. Direct numerical simulation and particle image velocimetry were used to investigate the fluid flow characteristics inside a 2 × 2 cell at Reynolds numbers that range between 100 and 1000. It was found that the flow in this geometry moves parallel to and also rotates between the spacer filaments and that the rate of rotation increases with Reynolds number. The flow mechanisms, transition process and onset of turbulence in a spacer-filled channel are investigated including the use of the velocity spectra at different Reynolds numbers. It is found that the flow is steady for Re < 200 and oscillatory at Re ～ 250 and increasingly unsteady with further increases in Re before the onset of turbulent flow at Re ～ 1000.     

Experimental investigation of laminar flow of a liquid with a free surface in triangular-shaped channels

Results are given of an experimental study of laminar flow of a liquid in triangular-shaped open channels with tangential frictional stress at the free surface. Experiments were carried out when the liquid flow in inclined triangular-shaped channels had Reynolds numbers R < 10 and the working range of Reynolds numbers of the approach air stream was R = (1.6–3.6)·10⁴. The data are presented in relative coordinates as a dependence of the hydraulic resistance coefficient of the liquid on the tangential frictional stress at the free surface. It is shown that with an increase of the tangential frictional stress the hydraulic resistance coefficient considerably increases.Translated from Izvestiya Akademii Nauk SSSR, Mekhanika Zhidkosti i Gaza, No. 1, pp. 168–170, January–February, 1977.     

A study of thin water film flow down an inclined plate without and with countercurrent air flow

Characteristics of thin water film flow down an inclined plane surface without as well as with superposed countercurrent air flow were studied experimentally. Three different angles of inclination of the channel with horizontal were investigated. At each inclination angle, five film Reynolds numbers were studied. Experiments were performed beginning at zero air flow and then increasing the air flow rate in steps until water entrainment occurred. Visual observation of the film surface was carried out as were film thickness measurements by means of capacitance probes. Results presented include mean film thickness and distribution, frequency spectra and propagation velocities of interfacial disturbances, and the incipience condition for entrainment of water from the film into the air stream.