Natural convection in rectangular enclosures with adiabatic fins attached on the heated wall

The heat transfer by natural convection in vertical and inclined rectangular enclosures with fins attached to the heated wall is numerically studied using the energy and Navier-Stokes equations with the Boussinesq approximation. The range of study covers 10⁴Ra2×10⁵,A=H/L=2.5 to ,B=l/L=0 to 1,C=h/L=0.25 to 2 andPr=0.72. The inclination angle from the vertical was from 0 to 60 degree. The variation of the local Nusselt numberNu _loc along the enclosure height and the average Nusselt numberNu as a function ofRa are computed. Streamlines and isotherms in the enclosure are produced. The results show thatB is an important parameter affecting the heat transfer through the cold wall of the enclosure. The heat transfer is reduced for decreasingC and it passes from a maximum for an inclination angle. The results show that the heat transfer can generally be reduced using appropriate geometrical parameters in comparison with a similar enclosure without fins.Die Wärmeübertragung bei freier Konvektion in vertikalen und geneigten rechtwinkligen Behältern mit Rippen an den beheizten Wänden wird unter Verwendung der Energie- und Navier-Stokes-Gleichungen sowie der Boussinesq-Approximation numerisch untersucht. Der Bereich der Studie liegt bei 10⁴Ra2·10⁵,A=H/L=2,5 bis ,B=l/L=0 bis 1,C=h/L=0,25 bis 2 undPr=0.72. Der Neigungswinkel der Wand liegt zwischen 0 und 60 Grad. Die Veränderung der lokalen Nusselt-Zahl entlang der Höhe der Behälterwände und die mittlere Nusselt-Zahl in Abhängigkeit derRa-Zahl werden berechnet. Strömungslinien und Isothermen werden im Behälter erzeugt. Die Ergebnisse zeigen, daßB ein wichtiger Parameter für die Wärmeübertragung an der nicht beheizten Wand des Behälters ist. Die übertragene Wärmemenge verringert sich mit abnehmendemC und durchschreitet ein Maximum für eine bestimmte Wandneigung. Die Ergebnisse zeigen, daß im Vergleich zu einer Anordnung ohne Rippen, die Wärmeübertragung bei geeigneten geometrischen Parametern allgemein reduziert werden kann.     

Natural convection in inclined rectangular enclosures with perfectly conducting fins attached on the heated wall

The natural convection heat transfer in inclined rectangular enclosures with perfectly conducting fins attached to the heated wall is numerically studied. The parameters governing this problem are the Rayleigh number (10²≤Ra≤2×10⁵), the aspect ratio of the enclosures (2.5≤A=H′/L′≤∞), the dimensionless lengths of the partitions (0≤B=?′/L′≤1), the aspect ratio of micro-cavities (A≤C=h′/L′≤0.33), the inclination angle (0≤φ≤60^°) and the Prandtl number (Pr=0.72). The results indicate that the heat transfer through the cover is considerably affected by the presence of the fins. At low Rayleigh numbers, the heat transfer regime is dominated by conduction. When B≈0.75 and C≈0.33, the heat transfer through the cold wall decreases considerably. This trend is enhanced when the enclosure is inclined. Useful engineering correlations are derived for practical applications.     

Laminar flow and mass transfer in channels with a porous bottom wall and with fins attached to the top wall

 The steady incompressible, viscous, two- dimensional flow of a solution in a channel was considered. The bottom wall was porous and the fins were attached to the top wall. Employing control volume approach, a computer program based on SIMPLE algorithm was developed. Computations were carried out to investigate the effects of the inlet Reynolds number, the fin length, the suction Reynolds number and the slip coefficient on the flow structure and the concentration distribution. It was observed that the thickness of concentration boundary layer increases in the flow direction. The concentration on the porous wall and the concentration boundary layer thickness decrease with increasing fin length, the slip coefficient and the inlet Reynolds number. These results show that fins attached to the upper wall of the channel can be utilized to reduce the concentration polarization and hence improve the effectiveness of the separation process. Received on 24 February 1999     

Numerical analysis of laminar natural convection in enclosures with fins attached to an active wall

Vertical enclosures with conducting fins attached to the cold wall were considered. Side walls were kept at constant but different temperatures, while horizontal top and bottom walls were insulated. A conjugate formulation was used for the mathematical formulation of the problem, and a computer program based on the control volume approach and the SIMPLE algorithm was developed. Computations were performed to investigate the effects of the fin configuration and Rayleigh number on the flow structure and heat transfer. It was observed that the heat transfer rate through an enclosure can be controlled by attaching fins to the wall(s) of the enclosure. At low Rayleigh numbers (conduction controlled regime), the heat transfer rate increases with the increasing number of fins and the fin length. However, at higher Rayleigh numbers (convection dominant regimes), the heat transfer rate can be decreased or increased by properly choosing the number of fins and the fin lengths. Received on 07 April 1997     

EXPERIMENTAL STUDY OF MEASUREMENT FOR DISSIPATION RATE SCALING EXPONENT IN HEATED WALL TURBULENCE

Experimental investigations have been devoted to the study of scaling law of coarse-grained dissipation rate structure function for velocity and temperature fluctuation of non-isotropic and inhomogeneous turbulent flows at moderate Reynolds number. Much attention has been paid to the case of turbulent boundary layer, which is typically the non-istropic and inhomogeneous trubulence because of the dynamically important existence of organized coherent structure burst process in the near wall region . Longitudinal velocity and temperature have been measured at different vertical positions in turbulent boundary layer over a heated and unheated flat plate in a wind tunnel using hot wire anemometer. The influence of non-isotropy and inhomogeneity and heating the wall on the scaling law of the dissipation rate structure function is studied because of the existence of organized coherent structure burst process in the near wall region . The scaling law of coarse-grained dissipation rate structure function is foun     

Experimental confirmation of the propulsion of marine vessels employing guided flexural waves in attached elastic fins

This paper describes the results of the first experimental verification of the idea of wave-like aquatic propulsion of manned marine vessels first published by the first author in 1994. The idea is based on employing the unique type of guided flexural elastic waves propagating along edges of immersed wedge-like structures attached to a body of a small ship or a submarine as keels or wings and used for the propulsion. The principle of employing such guided flexural waves as a source of aquatic propulsion is similar to that used in nature by stingrays. It is vitally important for the application of this idea to manned vessels that, in spite of vibration of the fins, the main body of the craft remains undisturbed as the energy of guided elastic waves is concentrated away from it. The main expected advantages of this new propulsion method over the existing ones, e.g. jets and propellers, are the following: it is quiet, and it is environmentally friendly and safe for people and wildlife. To verify the idea experimentally, the first working prototype of a small catamaran using the above-mentioned wave-like propulsion via the attached rubber keel has been built and tested. The test results have shown that the catamaran was propelled efficiently and could achieve the speed of 36 cm/s, thus demonstrating that the idea of wave-like propulsion of manned craft is viable. The reported proof of the viability of this idea may open new opportunities for marine craft propulsion, which can have far-reaching implications.     

Experimental study on the pressure drop and heat transfer characteristics of tubes with internal wave-like longitudinal fins

Experiments were performed to determine the heat transfer and pressure drop characteristics in the entrance and fully developed regions of tubes with internal wave-like longitudinal fins. The test tube has a double-pipe structure, with the inner tube as an insertion. The wave-like fins are in the annulus and span its full width. Experiments were conducted for two cases: one with the inner tube blocked (no air flowing through it) and the other with the inner tube unblocked. The outer tube was electrically heated. Local and average heat transfer coefficients and friction factors were measured. The friction factor and Nusselt number correlations in the fully developed region were obtained in the Reynolds number range of 9×10² to 3.5×10³. It has been found that the wave-like fins enhance heat transfer significantly with the blocked case being superior. In addition, the in-tube heat transfer process is characterized by an earlier transition from laminar to turbulent flow and Reynolds number-dependent thermal entrance length. Received on 12 May 1998     

A computational study of vortex rings interacting with a constant-temperature heated wall

This study is motivated by understanding the connections between the vortical structures in impinging jets and the wall heat transfer. Of particular interest are: (1) examining how the stage of evolution of vortex pairing in the jet might influence the wall heat transfer, and (2) establishing correlations between the vortex characteristics and the Nusselt number (Nu) distribution. To this end, CFD simulations are conducted of three simplified model problems involving the interaction of isolated axisymmetric vortex rings with a flat, constant-temperature, heated wall. The cases represent three scenarios of vortex-wall interaction: before (Case I), during (Case II) and after (Case III) pairing. The results show that when two vortices concurrently interact with the wall and undergo pairing (Case II), a significant instantaneous enhancement in Nu is attained in comparison to that associated with a single vortex interacting with the wall (Cases I and III). However, Case II also leads to the largest subsequent decay in Nu enhancement due to the formation of a particularly strong secondary vortex. In all cases, a deterioration in Nu, relative to unsteady diffusion, is observed simultaneously with the enhancement. Notwithstanding this deterioration, the net effect of vortex-wall interaction on the heat transfer remains positive with Case II producing the highest heat transfer rate. An analysis is conducted to establish the connection between the instantaneous maximum and minimum Nu, the circulation and the radial and the wall-normal location of the core-centers of the vortices, the thermal boundary layer thickness, the boundary layer separation location and the wall shear stress.     

Experimental study on the forced convective flow in a channel with heated blocks in tandem

This study experimentally examines the forced convective flow over two sequentially heated blocks mounted on one principal wall of a channel. The experiments, involving mass transfer, were carried out via the naphthalene sublimation technique (NST). By virtue of the analogy between heat and mass transfer, the results can then be converted to determine the heat transfer. In the experiments, the block spacings were set at 2, 4, 6, 8, 12, 16, and 22 and the Reynolds numbers were set at 1300 and 10⁴ which correspond to the laminar and the turbulent convective flow cases, respectively. Results show that the Sherwood number increases or decreases monotonically along the block surfaces in the laminar convection cases; while the hump and sharp increase in the Sherwood number can be found in the turbulent convection cases. This is attributed to the reattachment of the separating bubble and the flow impingement, respectively. Comparison between the experimental and numerical results is made and the effect of the block spacing on heat transfer is discussed.     

Laminar natural convection of Cu-water nanofluid in concentric annuli with radial fins attached to the inner cylinder

Laminar natural convection of Cu-water nano-fluid between two horizontal concentric cylinders with radial fins attached to the inner cylinder is studied numerically. The inner and outer cylinders are maintained at constant temperature. The governing equations in the polar two-dimensional space with the respective boundary conditions are solved using the finite volume method. The hybrid-scheme is used to discretize the convection terms. In order to couple the velocity field and the pressure in the momentum equations, the well known semi-implicit method for pressure linked equation reformed algorithm is adopted. Using the developed code, a parametric study is undertaken, and the effects of the Rayleigh number, Number of fins, length of the fins and the volume fraction of nano-particles on the fluid flow and heat transfer inside the annuli are investigated. In this study, two cases with different number of fins are considered. It is observed from the results that the average Nusselt number increases with increasing both the Rayleigh number and the volume fraction of the nano-particles. Moreover, the average Nusselt number decreases by increasing the fins’ length and the number of fins. Heat transfer rate increases by increasing the fins’ length at all Rayleigh numbers, but it increases by increasing the number of fins at high Rayleigh numbers.     

Experimental study on critical heat flux in bilaterally heated narrow annuli

Critical heat flux (CHF) experiments using deionized water as working fluid have been conducted in a range of pressure from 0.6 to 4.2 MPa, mass flow velocity from 60 to 130 kg/m² s and wall heat flux from 10 to 90 kW/m² for vertical narrow annuli with annular gap sizes of 0.95 and 1.5 mm. We found that the CHF, occurring only on the inside tube, or on the outside tube or on both tubes of the annular channel, depends on the heat flux ratio between surfaces of the outside and inside tubes. The CHF, occurring on the surface of the inside tube, reaches the maximum value under the pressure of 2.3 MPa while it occurring on the surface of the outside tube keeps increasing with the increase of the pressure. The CHF, occurring on the inside or outside tubes, increases with the increase of the mass flow velocity and the annular gap size; and decreases with the increase of critical quality and the other tube wall heat flux. Empirical correlations, which agree quite well with the experimental data, have been developed to predict the CHF occurring on surfaces of the inside or outside tubes of the narrow annular channel on the conditions of low pressure and low flow.     

Structure of turbulent two-dimensional channel flow with strongly heated wall

A laser Doppler velocimeter and a resistance thermometer were used to study velocity and temperature statistics in a strongly heated turbulent two-dimensional channel flow, with the wall temperature up to 700 °C and a Reynolds number of 14,000. Normalized mean velocity and mean temperature profiles were not significantly affected by the wall heating. Turbulent intensities of temperature fluctuation were also insensitive to the heat flux. However, turbulent intensities of velocity fluctuation were suppressed in the region away from the wall, whereas those near the wall were not changed noticeably by the wall heating. This phenomenon was explained by the balance of three parameters: turbulent production, viscous dissipation and intermittency.     

Optimization of a buoyancy chimney with a heated ribbed wall

Heat and mass transfer in natural convection vertical channels was investigated by means of two-dimensional CFD simulations aided by optimization algorithms. The channel was immersed in air, enclosed between an adiabatic smooth wall and an isothermally heated ribbed wall. The ribs were perpendicular to the fluid flow and their height, width, pitch, thermal conductivity and lateral wall inclination were variable. Also the smooth heated wall channel was studied and compared with the ribbed one. The existence of an optimal channel width for a given channel height and rib geometry was shown. A sensitivity analysis was carried out for the ribbed and the smooth channels. Optimization was applied to the ribbed channel problem in order to maximize the heat and the mass transfer through a multi-objective genetic algorithm. It was found that the presence of the ribs penalizes the channel performance so that no ribbed channel over-performed the smooth one.     

Experimental observations of the thermal flow around a square obstruction on a vertical wall in a differentially heated cavity

The transient thermal boundary layer flow around a square obstruction placed at the middle of the hot wall in a differentially heated cavity is visualized using a shadowgraph technique. The results show that the thermal boundary layer flow, which is blocked by the obstruction, firstly forms an intrusion head under the obstruction (the lower intrusion head). Subsequently, the lower intrusion head bypasses the obstruction and reattaches to the down-stream boundary. During the reattachment process, a more complicated flow is induced, and eventually both the lower intrusion head and the thermal boundary layer destabilize. After the lower intrusion head is convected away, the thermal boundary layer flow re-stabilizes. At the quasi-steady state, the thermal boundary layer forms a double-layer structure, which is split into two sections by the obstruction. It is demonstrated that both the transient processes and the quasi-steady state flow structures of the thermal boundary layer are significantly altered by the obstruction in comparison with the case without the obstruction.     

Experimental and numerical study on the self-stress design of tensegrity systems

Tensegrity systems as kinematically and statically indeterminate pin-jointed systems are characterized by mechanisms and self-stress states. Unlike the other reticulated systems, in tensegrity systems, unilateral behavior of cables causes some problems in determining the basis of compatible self-stress states. At the present study, self-stress design of tensegrity systems is presented. Experimental study on two 3×3×0.7 m tensegrity grids was conducted to verify the accuracy and validity of the numerical method. Using supporting constraints, an effective method for the implementation of self-stress states in a much reduced number of stages is proposed and calibrated. Considering the results of the present study, the self-stress design of these systems can be improved to obtain specific desired behavior.     

Tracking of coherent thermal structures on a heated wall.

The temporal evolution of a thermal pattern observed on a heated wall by infrared camera is correlated with the propagation velocity of the thermal perturbations calculated by DNS. In the experiment the propagation velocity was measured by using PIV-based analysis of infrared images of the thermal pattern on the wall. To verify the experimental technique of image analysis, a sequence of synthetic images, simulating thermal patterns on the wall, was generated from the DNS solution, and the convective velocity was evaluated. It was found that the convective velocity of thermal structures obtained by PIV-based analysis of the experimental and synthetic images was in relatively good agreement with that calculated from the DNS solution. The present study confirmed that for a high Prandtl number fluid (water) the propagation velocity of the thermal perturbations is only about half of the convective velocity of the velocity perturbations. It was also found that the convection velocity observed for hot spots is distinctly lower than that for the cold spots.     

Experimental study of parallel and inclined turbulent wall jets

An experimental study of the flow field in a two-dimensional wall jet has been conducted. All measurements were carried out using hot-wire anemometry. The experimental facility has a rectangular slot nozzle of high aspect ratio l/b = 100 (where l and b are the length and height slot, respectively). Mean velocities and Reynolds stresses were determined with three nozzle Reynolds numbers (Re = 1 × 10⁴, 2 × 10⁴ and 3 × 10⁴) and four different inclination angles between the wall and the flow velocity at the nozzle (β = 0°, 10°, 20° and 30°). Results indicate that all wall jets are self-preserving in the developed region. Normal to the wall two regions can be identified: one similar to a plane free jet and the other similar to a boundary layer. Downstream the interaction between these two regions creates a mixed or third region. The logarithmic region increases with the distance from the nozzle and with the Reynolds number. For the inclined wall jet, the spreading rate expressed in terms of jet half-width or maximum velocity decay with respect to the streamwise distance, asymptotes to a linear law. The streamwise locations where the jet becomes self-similar are farther from the exit than in parallel wall jet. The slope of both half-width and maximum velocity decay in the developed region are affected by both wall jet inclination angle and nozzle exit Reynolds number.     

Experimental study of rotation effect on film cooling over the flat wall with a single hole

A new rotating test rig was set up to investigate the rotation effect on the film cooling over the flat wall. A simple flat blade with an inclined 30° film hole, which is parallel to the hot mainstream, was installed. And different rotation orientations were selected to simulate the blade pressure or suction side of a turbine blade. A steady liquid crystal technique was applied to obtain detailed distribution of the temperature over the blade surface. And the average adiabatic film cooling effectiveness of the area adjacent to the film hole was selected to evaluate the cooling effect. Five different rotational speeds, i.e., 0, 300, 500, 800, 1000 r/min, were considered. Experimental results indicate that the film trajectory could bend under the rotating condition. With the increase of the rotational speed, on the pressure side, the film trajectory inclines centripetally firstly and then centrifugally; whereas, on the suction side the film trajectory bends centrifugally. On the other hand, as the rotational speed increases, the cooling effect is improved firstly and then worsened when Ω > 500–600 r/min on the pressure side. On the suction side, however, the cooling effect is not sensitive to the rotational speed.     

Measurement of the aerodynamic forces on a small particle attached to a wall

Using operating principles similar to that applied in atomic force microscopes, we have developed a novel measuring method to study the aerodynamic forces, in particular the lift and drag force, acting on a small particle attached to a wall and immersed in a linear shear flow. Results thus far have shown that the system is capable of measuring both the minute aerodynamic lift and drag forces that a particle experiences as a result of the flow.C. Muthanna has also published under the name C. M. Kolera

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