Analysis of heat transfer and pressure drop characteristics in an offset strip fin heat exchanger

A steady-state three-dimensional numerical model was used to study the heat transfer and pressure drop characteristics of an offset strip fin heat exchanger. Water was the heat transfer medium, and the Reynolds number Re_dh ranged from 10 to 3500. Variations in the Fanning friction factor f and the Colburn heat transfer factor j relative to Re_dh were observed. General correlations for the f and j factors were derived, and these could be used to analyze fluid flow and heat transfer characteristics of offset strip fins in the laminar, transition, and turbulent regions. Finally, three performance criteria (j/f, j/f^1/3, and JF) were adopted, and the best performance criteria for the cases Pr = 7 and Pr = 50 were chosen to be JF and j/f^1/3, respectively.     

Effect of radiation in a radial heat sink under natural convection

A radial heat sink was investigated, considering both natural convection and radiation. Experiments were performed to validate numerical results and a close agreement was found. The effect of radiation on total heat transfer was examined by varying emissivity, and it was found that the maximum radiation contribution was 27%. The radial heat sink was optimized to maximize thermal performance. The thermal performance was enhanced by 12.3% while the mass increased by 20%. Radiation (emissivity) had a significant effect on the optimum long fin length when only thermal performance was considered. The average thermal resistance decreased by 8.7% for the optimized model with the same mass as the reference model, and the effect of radiation on total heat transfer was greater than the effect of natural convection.     

Two-dimensional hyperbolic heat conduction in an orthotropic medium

Many natural and man-made materials have the properties of varying thermal conductivity with directions. In the present work, the very short-time temperature response characteristics are examined by using a hyperbolic heat conduction model in an orthotropic medium when an axially symmetric heat flux with a temporal profile, which is either continuous or activated for period Δt, is applied. The ratio of the thermal conductivities is one of the important parameters to be considered. The non-Fourier temperature responses in the orthotropic medium are compared with those in the isotropic case. The Fourier's results are also included and compared with the non-Fowrier's results.     

Frost behavior on a fin considering the heat conduction of heat exchanger fins

A mathematical model is proposed for predicting frost behavior on a heat exchanger fin under frosting conditions, taking into account fin heat conduction. The change in the three-dimensional airside airflow caused by frost growth is reflected in this model. The numerical estimates of frost thickness are consistent with experimental data, with an error of less than 10%. Due to fin heat conduction, frost thickness decreases exponentially toward the fin tip, while considerable frost growth occurs near the fin base. When a constant fin surface temperature is assumed, the predicted frost thickness was larger by more than 200% at maximum, and the heat flux by more than 10% on average, compared to results obtained with fin heat conduction taken into account. Therefore, fin heat conduction could be an essential factor in accurately predicting frost behavior. To improve prediction accuracy under the assumption of constant fin surface temperature, the equivalent temperature (for predicting frost behavior) is defined to be the temperature at which the heat transfer rate neglecting fin heat conduction is the same as the heat transfer rate with fin heat conduction taken into consideration. Finally, a correlation for predicting the equivalent temperature is suggested.     

EFFECT OF GEOMETRIC PARAMETERS ON VENTILATION PERFORMANCE IN A DRY ROOM

The characteristics of moisture ventilation in a dry room are studied numerically. The effects of three important parameters: the positions of outlets, the aspect ratio (β) of horizontal section and the air exchange rate (N), are analyzed in terms of the scales of ventilation efficiency. Ventilation performance is evaluated by varying the aspect ratio and the air exchange rate for the four different outlet arrangements. It is found that the ventilation performance is improved when the aspect ratio in the transverse arrangement of outlets is decreased. Ventilation efficiency reaches its highest value when β is set to be 4 in the longitudinal arrangement of outlets. Regardless of the aspect ratio, the upper arrangement of the outlets shows a better ventilation performance than the lower arrangement. For all geometrical models and aspect ratios investigated in this study, the increase of the air exchange rate for improving the ventilation performance is only valid up to 60 h^-1.     

Dimensionless correlations of frost properties on a cold cylinder surface

We propose dimensionless correlations for frost properties on a cold cylinder surface. Frosting experiments were performed while changing various frosting parameters such as the air temperature, cold cylinder surface temperature, air velocity, and absolute humidity. The experimental data showed that a uniform frost layer grew around the circumference of the cylinder at a high air velocity. Dimensionless correlations for the thickness, density, and surface temperature of the frost layer, and for the heat transfer coefficient were obtained as functions of the Reynolds number, Fourier number, absolute humidity, and dimensionless temperature. The applicable ranges of these correlations are Reynolds number of 700–3000 (air velocities of 0.5–2.0 m/s), Fourier number of 56.8–295.7 (operating time of 0–100 min), absolute humidity of 0.00280–0.00568 kg/kg_a, air temperatures of 3–9 °C, and cold cylinder surface temperatures of ?32 to ?20 °C. The proposed correlations agreed with the experimental data within an error of 15%.