S. Baskaya

Parametric study of natural convection heat transfer from horizontal rectangular fin arrays

Abstract A systematic theoretical investigation of the effects of fin spacing, fin height, fin length and temperature difference between fin and surroundings on the free convection heat transfer from horizontal fin arrays was carried out. The three-dimensional elliptic governing equations were solved using a finite volume based computational fluid dynamics (CFD) code. Preliminary simulations were made for cases reported in the literature. After obtaining a good agreement with results from the literature a large number of runs were performed for a detailed parametric study. It has been shown that it is not possible to obtain optimum performance in terms of overall heat transfer by only concentrating on one or two parameters. The interactions among all the design parameters must be considered. Results are presented in graphical form together with optimum values and correlations, and compared with available experimental data from the literature.

Fuzzy Modeling of Performance of Counterflow Ranque-Hilsch Vortex Tubes with Different Geometric Constructions

In this article, we present the development of a fuzzy expert system (FES) for fuzzy modeling of the performance of counterflow Ranque-Hilsch vortex tubes for different geometric constructions. Experimental values were obtained from a detailed experimental investigation. With these experimental values, FES models of the Ranque-Hilsch vortex tube behavior were designed using the MATLAB 6.5 fuzzy logic toolbox in Windows XP running on an Intel 3.0-Ghz PC. For this process P, N, ξ, and L/D were chosen as input and ΔT h , ΔT c , ΔT as output parameters. FES results agree well with experimental data. It was found that the coefficient of multiple determination (R 2 value) between the actual and fuzzy predicted data is ΔT h = 0.9801, the R 2 value for ΔT c values is 0.9841, and the R 2 value for ΔT values is 0.9748.

Numerical analysis of Grashof number, Reynolds number and inclination effects on mixed convection heat transfer in rectangular channels

Abstract Mixed convection heat transfer in rectangular channels has been investigated computationally under various operating conditions. The lower surface of the channel is subjected to a uniform heat flux, sidewalls are insulated and adiabatic, and the upper surface is exposed to the surrounding fluid. Solutions were obtained for Pr=0.7, inclination angles 0° ≤ θ ≤ 90°, Reynolds numbers 50 ≤ Re ≤ 1000, and modified Grashof numbers Gr = 7.0×10 5 to 4.0×10 6 . The three-dimensional elliptic governing equations were solved using a finite volume based computational fluid dynamics (CFD) code. From a parametric study, local Nusselt number distributions were obtained and effects of channel inclination, surface heat flux and Reynolds number on the onset of instability were investigated. Results obtained from the simulations are compared with the literature and a parallel conducted experimental study, from which a good agreement was observed. The onset of instability was found to move upstream for increasing Grashof number. On the other hand, onset of instability was delayed for increasing Reynolds number and increasing inclination angle.

The radial spread and axial decay of temperature in turbulent condensing jets

Radial and axial time-averaged temperature distributions in turbulent steam jets discharged into ambient air under unchoked/choked and saturated/superheated nozzle exit conditions have been measured with the aid of a fine thermocouple. Centreline decay and half-width spreading rates were calculated for these two-phase (liquid and vapour), two-fluid (air and water) jets and compared with data from the literature for non-condensing jets. Condensing jets exhibit higher spreading rates but much lower decay rates compared to non-condensing jets. A significant temperature increase due to condensation was observed. This increase in temperature became smaller with increased superheat and hence reduced condensation. Preliminary computational fluid dynamics (CFD) simulations were also performed using various simplifying assumptions.

Experimental and Numerical Study on Thermoelectric Generator Performance Applied to a Condensing Combi Boiler

In this study, thermoelectric generator (TEG) adaptation in a condensing combi boiler was investigated. The first part of the study comprises experimental analysis, performance tests of a commercially available TEG, and efficiency tests of a condensing combi boiler without TEG adaptation. In the second part of the study, a numerical analysis was carried out to determine the locations of TEGs inside the heat cell. According to the results of the performance tests, approximately 16 V open circuit voltage was generated under 201°C temperature difference. The electrical efficiency of the tested TEG is calculated to be 4.5%. Natural gas was used as fuel in the efficiency tests of the condensing combi boiler and the thermal efficiency was calculated to be 92.3% for 40–60°C water supply and return temperatures. Numerical study showed that the highest temperature difference could be obtained at the upper side of the heat cell with a value of 270°C. The hot side of the TEG module was heated from the inner surface of the heat cell via conduction through the wall, and the water channel was used as the heat sink. Therefore, TEGs were located inside the water channel of the heat cell, with a modification process on the heat cell itself. The performance tests and numerical results show that it is possible to integrate TEGs into the heat cell and decrease the electricity consumption or use combi boilers without an electricity network connection with a properly designed TEG integrated heat cell.

Design Analysis of Impinging Jet Array Heat Transfer From a Surface With V-Shaped and Convergent–Divergent Ribs by the Taguchi Method

In this work, the effects of the impingement plate geometry, Reynolds number, jet-plate distance-to-jet diameter ratio, and the jet diameter-to-rib height ratio on the impingement jets were investigated using the Taguchi experimental design method. The heat transfer measurements over a surface with V-shaped ribs (V-SR) and convergent-divergent shaped ribs (CD-SR) by a circular impinging jet array were investigated using a thermal infrared camera. Both V-SR and CD-SR configurations, with an angle of 45°, were considered. Both projected and total Nusselt numbers were considered as performance statistics. After the data on each quality characteristic were obtained from the orthogonal arrays, a main effect analysis and analysis of variance (ANOVA) were conducted to determine the parameters that had significant effects on these quality characteristics. The L32 (21 × 43) orthogonal array was selected as an experimental plan for the four parameters. The Taguchi method was applied with an optimization process to reach the maximum heat transfer. Experimental results validated the suitability of the proposed approach. The best heat transfer performance was obtained with the V-SR arrangements.

Transient Turbulent Flow and Heat Transfer Phenomena in Plate-Fin Type Cross-Flow Heat Exchanger

In this article, a transient performance of a plate-fin cross-flow heat exchanger with convergent–divergent longitudinal vortex generators is investigated. The effect of flow geometry is taken into account to analyze the transient forced convection heat transfer in a designed heat exchanger. The time-dependent Nusselt number and dissipation energy criterion are experimentally measured in 4- and 8-kW heater powers for various Reynolds numbers between 42,000 and 60,000 for the hot and cold fluids. In order to present the quality of the heat exchanger, the general empirical equations of the time-dependent Nusselt number and friction factor were derived as a function of the Reynolds number corresponding to fin geometry parameters. Following this, the transient behavior of the heat exchanger according to the change in the inlet and outlet temperatures of the hot and cold fluids was analyzed. The results showed that the variations of the time-dependent dissipation energy criterion increase with the increase in the Reynolds number. The appropriate correlations are proposed to predict the heat transfer and friction characteristics of the transient performance for the presented configuration, which indicates the designed heat exchanger has good heat conduction.

Investigation of room air flow regarding effects of occupants, inlet/outlet locations, inlet velocity, and winter/summer conditions

The air distribution inside a room containing one person and office furniture under different inlet/outlet and summer/winter configurations was investigated numerically. The steady two-dimensional equations of conservation were solved under defined boundary conditions using computational fluid dynamics. Results were presented in the form of velocity vectors and temperature contours, in addition to, quantitative velocity and temperature distributions. Comfort conditions and effects due to the person and the other occupants on the airflow were examined. The present numerical predictions show that occupants significantly alter the indoor air movement and hence affect comfort conditions.

Buoyancy-Induced Flow Through a Narrow Chamber Containing an Internal Heat Source: Comparison of Experimental Measurements and Numerical Simulations

Natural convection ventilation of a rectangular chamber containing a heated rectangular body has been investigated both experimentally and computationally. The heated square blockage within the partial enclosure was adjacent to the adiabatic lower surface of the chamber and the three exposed surfaces of the block were such as to give a constant heat flux. The air inlet and outlet were located at the bottom and top of the chamber vertical walls respectively, and were of equal area. Velocity measurements were performed using laser Doppler anemometry (LDA) with a one-component He–Ne laser connected to a burst spectrum analyser (BSA). Detailed velocity profiles were measured at the inlet, outlet and at several locations inside the chamber. Temperatures of the heated air at the outlet were measured with a chromel–alumel (type K) bare wire thermocouple probe. Three-dimensional laminar and turbulent (k–e model) numerical simulations were obtained by solving the governing equations using the computational fluid dynamics (CFD) code PHOENICS. Comparisons of experimental and computational results showed very good agreement in most of the flow field.

LDA measurements and numerical simulation of the induced flow through a rectangular chamber containing a vertical cylindrical heat source

Experimental (LDA) and computational (CFD) investigations of steady, laminar natural convection in a horizontally vented chamber containing a vertical heated cylinder at the center are reported. The heated cylinder was located on the lower adiabatic wall of the chamber and was 2/3 the height of the chamber. The cylinder surfaces were maintained at a constant temperature. The chamber inlet and outlet were located at the bottom and top of facing vertical walls, respectively. In this study, experiments were conducted using laser-Doppler anemometry (LDA) with a two-component Ar-Ion laser connected to two burst spectrum analyzers (BSA). Velocity profiles (of both vertical and horizontal velocity components) were measured at the inlet, outlet and at other locations inside the chamber. For the numerical simulation (3-D, steady-state, laminar) the governing equations were solved using the computational fluid dynamics code PHOENICS. Comparison of the experimental data with computational results showed satisfactory agreement and the importance that LDA measurements have in verifying CFD simulations.