Jiin Yuh Jang

Numerical and experimental studies of threedimensional plate-fin and tube heat exchangers

Abstract Fluid flow and heat transfer over a multi-row (1–6 rows) plate-fin and tube heat exchanger are studied numerically and experimentally. Fluid flow is incompressible, three-dimensional and laminar. The effects of different geometrical parameters such as tube arrangement, tube row numbers and fin pitch (8–12 fins per inch) are investigated in detail for the Reynolds number (based on the fin spacing and the frontal velocity) ranging from 60 to 900. The average heat transfer coefficient of staggered arrangement is 15%–27% higher than that of in-lined arrangement, while the pressure drop of staggered configuration is 20%–25% higher than that of in-lined configuration. Average Nusselt number is decreased as the number of tune row is increased from 1 to 6. The number of tube row has a small effect on the average heat transfer coefficient as the row numbers became greater than 4. The numerical results for the average heat transfer coefficient and pressure drop agree well with the experimental measurements.

Local heat transfer measurements of plate finned-tube heat exchangers by infrared thermography

Abstract An experimental study is performed using an infrared thermovision to monitor temperature distribution over a plate-fin surface inside the plate finned-tube heat exchangers. The differentiation of the temperature function is derived to determine the local convective heat transfer coefficients on the tested fin, using a local element lumped conduction equation included the convective effect on the boundaries with experimental data. It is disclosed that the infrared thermography is capable of rapidly detecting location and extent of transition and separation regions of the boundary layer over the whole surface of the tested models. Through the comparison of the test results on the strategy region of the in-line and staggered arrangements, it is more easy to understand or interpret the detailed dynamic phenomena of flow existed in the heat exchangers. In addition, the experimental results demonstrate that the averaged heat transfer coefficient of staggered configuration is 14–32% higher than that of in-lined configuration

Experimental and 3-D numerical analysis of the thermal-Hydraulic characteristics of elliptic finned-tube heat exchangers

Abstract Fluid flow and heat transfer over a 4-row. elliptic, finned-tube heat exchanger having an axis ratio of 2.83:1 are studied experimentally and numerically. Three types of finned-tube configurations have been investigated under dry and wet conditions for different values of inlet frontal velocity ranging from 2 to 7 m/s: two elliptic finned tubes with staggered and in-line arrangements and one circular finned tube with staggered arrangement. The experimental results indicate that the average heat transfer coefficient of an elliptic finned lube is 35–50% of the corresponding circular finned tube having the same tube perimeter, while the pressure drop for an elliptic finned-tube bank is only 25–30% of the circular finned-tube bank configuration. Three-dimensional numerical results of a laminar model for dry coils are also presented and are compared with the experimental data.

Technical Note A heat transfer and friction correlation for wavy fin-and-tube heat exchangers

A generalized heat transfer and friction correlation for wavy fin geometry is proposed from a total of 27 samples of fin-and-tube heat exchangers

Numerical analysis of heat transfer and fluid flow in a three-dimensional wavy-fin and tube heat exchanger

Abstract The effects of different geometrical parameters, including tube row numbers (1–4 rows), wavy angles (θ = 8.95°, 17.05°, 32.21°) and wavy heights (S = 0.751, 1.500 and 3.003 mm) are investigated in detail for the Reynolds number ReH (based on the fin spacing and the frontal velocity) ranging from 400 to 1200. Numerical results indicate that the row effect is less important in a wavy-fin as compared to plain-fin counterpart. It is also found that, for equal wavy height, both the average Nusselt number and pressure coefficient are increased as the wavy angle is increased; while for equal wavy angle, they are decreased as the wavy height is increased. The combination of (θ = 8.95°, S = 1.500 mm) gives the highest flow area goodness factor (j/ƒ). A comparison of the numerical results with the available experimental data is also presented.

A two-dimensional fin efficiency analysis of combined heat and mass transfer in elliptic fins

Abstract This paper presents a two-dimensional analysis for the efficiency of an elliptic fin under the dry, partially wet and fully wet conditions of a range of value for axis ratios, Biot numbers, and air humidifies. It is shown that the fin efficiencies increase as the axis ratio Ar is increased. For a given axis ratio Ar, the fin efficiency decreases as the fin height l ∗ or Biot number is increased. The conventional 1-D sector method overestimates the fin efficiency resulting in increasing error as the axis ratio Ar is increased. In addition, using experimentally determined heat transfer coefficients, it is found that both the fully dry and wet elliptic fin efficiencies are up to 4–8% greater than the corresponding circular fin efficiencies having the same perimeter.

Forced convection in a parallel plate channel partially filled with a high porosity medium

Abstract This paper presents a numerical study of non-Darcy effects on the fully developed forced convection parallel plate channel flow partially filled with a porous medium. The Navier-Stokes equations govern the fluid motion in the fluid region, while Darcy-Brinkman-Forchheimer model is assumed to hold within the porous media. The effect of thermal dispersion in the porous matrix is also considered. Inclusion of these effects significantly alters the velocity and temperature profile for those predicted by Darcy and Darcy-Brinkman models. It is shown that the Nusselt number strongly depends on the open space thickness ratio S, and a critical value of the porous layer thickness exists at which the Nusselt number reaches a minimum. The critical value of the porous layer thickness shifts to lower S as the Darcy number decreases. In addition, it is found that thermal dispersion effect is only significant for small values of open space thickness ratio S.

Effect of circuit arrangement on the performance of air-cooled condensers

Abstract An experimental study was carried out to investigate the effect of circuitry on the performance of wavy finned condensers. A total of eight arrangements were made and tested. The arrangements included six 1-circuit and two 2-circuit arrangements. For the one-circuit arrangement, the test results indicate that counter-cross flow would give better performance than other arrangements. However, heat conduction along the fins may offset the benefits of the counter-cross arrangement. This study has proposed two modifications to the counter-cross flow arrangement. For the two-circuit arrangement, a unique characteristic of “pressure gain” was observed when one circuit is completely condensed and the other is still in two-phase region.

The thermal-hydraulic characteristics of staggered circular finned-tube heat exchangers under dry and dehumidifying conditions

Abstract Fluid flow and heat transfer over four row circular finned-tube heat exchangers with staggered arrangement are studied experimentally and numerically. Two types of finned-tube configurations have been investigated under the dry and wet conditions for different values of inlet frontal velocity ranging from 1 to 6 m s −1 . The experimental results indicated that the sensible Colburn factor, J s , and for the friction factor, φ, the wet coils are, respectively 20% and 15% higher than that for the dry coils. The three-dimensional numerical results of laminar model for the dry coils are also presented. Conjugate convective heat transfer in the flow field and heat conduction in the circular fins are considered also. The numerical results for the streamline, isotherm, Nusselt number and fin efficiency are shown and compared with the experiments.

Buoyancy-induced inclined boundary layer flow in a porous medium resulting from combined heat and mass buoyancy effects

Abstract An implicit finite difference method is used to analyze the natural convection boundary layer flow in a saturated porous medium resulting from combined heat and mass buoyancy effects adjacent to an inclined surface. Both the streamwise and normal components of the buoyancy force are retained in the momentum equations. The present formulation permits the angles of from the horizontal. Numerical results indicate that, as the buoyancy ratio or inclination parameter increase, the surface heat and mass transfer rates increase. These results are compared with the approximate similarity solutions that are obtained by neglecting the normal component of the buoyancy force in the momentum equations. It is shown that the approximate similarity solutions may significantly underpredict the heat and mass transfer rates for small values of inclination parameter.