Nae-Hyun Kim

Air-Side Heat Transfer and Friction Correlations for Plain Fin-and-Tube Heat Exchangers With Staggered Tube Arrangements

This paper deals with heat exchangers having plain fins on a staggered array of circular tubes. Correlations are developed to predict the air-side heat transfer coefficient and friction factor as a function of the Reynolds number and geometric variables of the heat exchanger such as tube diameter, tube pitch, fin spacing, etc. A multiple regression technique was used to correlate 47 sets of heat exchanger data to develop the heat transfer and friction correlation. The correlations are applicable to heat exchangers having small diameter tubes (or large tube pitch to tube diameter ratio), whose performance previous correlations failed to predict adequately. The heat transfer correlation applicable to three or more row configuration predicts 94% of the data within {+-}20%, and the heat transfer correlation applicable to one- or two-row configuration predicts 94% of the data within {+-}20%. The friction correlation predicts 90% of the data within {+-}20%.

Nucleate pool boiling on structured enhanced tubes having pores with connecting gaps

Abstract In this study, pool boiling test results are provided for the structured enhanced tubes having pores with connecting gaps. The surface geometry of the present tube is similar to that of Turbo-B. Three tubes with different pore size (0.20, 0.23 and 0.27 mm) were manufactured and tested using R-11, R-123 and R-134a. The pore size which yields the maximum heat transfer coefficient varied depending on the refrigerant. For R-134a, the maximum heat transfer coefficient was obtained for the tube having 0.27 mm pore size. For R-11 and R-123, the optimum pore size was 0.23 mm. One novel feature of the present tubes is that their boiling curves do not show a ‘cross-over’ characteristic, which existing pored tubes do. The connecting gaps of the present tubes are believed to serve an additional route for the liquid supply and delay the dry-out of the tunnel. The present tubes yield the heat transfer coefficients approximately equal to those of the existing pored enhanced tubes. At the heat flux 40 kW/m 2 and saturation temperature 4.4°C, the heat transfer coefficients of the present tubes are 6.5 times larger for R-11, 6.0 times larger for R-123 and 5.0 times larger for R-134a than that of the smooth tube.

Heat Transfer and Friction Correlations for Wavy Plate Fin-and-Tube Heat Exchangers

This paper deals with heat exchangers having plate fins of herringbone wave configuration. Correlations are developed to predict the air-side heat transfer coefficient and friction factor as a function of flow conditions and geometric variables of the heat exchanger. Correlations are provided for both staggered and in-line arrays of circular tubes. A multiple regression technique was used to correlate 41 wavy fin geometries by Beecher and Fagan (1987), Wang et al. (1995) and Beecher (1968). For the staggered layout, 92% of the heat transfer data are correlated within {+-}10%, and 91% of the friction data are correlated within {+-}15%.

Condensation heat transfer of R-22 and R-410A in flat aluminum multi-channel tubes with or without micro-fins

Abstract In this study, condensation heat transfer tests were conducted in flat aluminum multi-channel tubes using R-410A, and the results are compared with those of R-22. The flat tubes have two internal geometries; one with smooth inner surface and the other with micro-fins. Data are presented for the following range of variables; vapor quality (0.1–0.9), mass flux (200–600 kg/m2s) and heat flux (5–15 kW/m2). Results show that the effect of surface tension drainage on the fin surface is more pronounced for R-22 than R-410A. The smaller Weber number of R-22 may be responsible. For the smooth tube, the heat transfer coefficient of R-410A is slightly larger than that of R-22. For the micro-fin tube, however, the trend is reversed. Possible reasoning is provided considering physical properties of the refrigerants. For the smooth tube, Webbs correlation predicts the data reasonably well. For the micro-fin tube, the Yang and Webb model was modified to correlate the present data. The modified model adequately predicts the data.

Forced convective boiling of pure refrigerants in a bundle of enhanced tubes having pores and connecting gaps

Abstract In this study, convective boiling tests were conducted for enhanced tube bundles. The surface geometry consists of pores and connecting gaps. Tubes with three different pore sizes (dp=0.20, 0.23 and 0.27 mm) were tested using R-123 and R-134a for the following range: 8 kg / m 2 s ⩽G⩽26 kg / m 2 s , 10 kW / m 2 ⩽q⩽40 kW / m 2 and 0.1⩽x⩽0.9. The convective boiling heat transfer coefficients were strongly dependent on heat flux with negligible dependency on mass flux or quality. For the present enhanced geometry (pores and gaps), the convective effect was apparent. The gaps of the present tubes may have served routes for the passage of two-phase mixtures, and enhanced the boiling heat transfer. The convective effect was more pronounced at a higher saturation temperature. More bubbles will be generated at a higher saturation temperature, which will lead to enhanced convective contribution. The pore size where the maximum heat transfer coefficient was obtained was larger for R-134a ( d p =0.27 mm ) compared with that for R-123 ( d p =0.23 mm ). This trend was consistent with the previous pool boiling results. For the enhanced tube bundles, the convective effect was more pronounced for R-134a than for R-123. This trend was reversed for the smooth tube bundle. Possible reasoning is provided based on the bubble behavior on the tube wall. Both the modified Chen and the asymptotic model predicted the present data reasonably well. The RMSEs were 14.3% for the modified Chen model and 12.7% for the asymptotic model.

MODELING AND VERIFICATION OF HEAT AND MOISTURE TRANSFER IN AN ENTHALPY EXCHANGER MADE OF PAPER MEMBRANE

In this study, heat and moisture transfer model of an enthalpy exchanger is proposed. With separately measured sorption constant and diffusion coefficient, the model predicts the heat and moisture transfer effectiveness of an enthalpy exchanger. Two sample enthalpy exchangers were tested at a KS condition to verify the model. The model predicts the heat transfer effectiveness within 4%, and the moisture transfer effectiveness within 10%. Pressure drop is predicted within 6%. The spacer fin efficiency for heat transfer was 0.11 to 0.13. The fin efficiency for moisture transfer, however, was negligibly small. For heat transfer, the conduction resistance to total thermal resistance was less than 1%. For moisture transfer, however, membrane resistance was dominant to convective moisture transfer resistance.

Analytic Prediction of the Friction and Heat Transfer for Turbulent Flow in Axial Internal Fin Tubes

An analytic model is developed to predict the friction factors and Nusselt numbers for turbulent flow in axial internal fin tubes. The present model uses the Law of the Wall and applies the logarithmic universal velocity and temperature profile to the interfin and core regions of the flow. The fin shape is assumed trapezoidal, and the fin parameters such as fin height, fin root thickness and fin tip thichness are determined from the tube dimensional data. Theoretically based friction and heat transfer equations are developed for internally finned tubes in an algebraic form

Evaporation Heat Transfer and Pressure Drop of R-410A in a 5.0 mm O.D. Smooth and Microfin Tube

R-410As evaporation heat transfer and pressure drop data are provided for a 5.1 mm O.D. microfin tube having 40 fins with 18° helix angle and 40° fin apex angle. Tests were conducted for a range of quality (0.2–0.6), mass flux (260–433 kg/m2s), heat flux (10–20 kW/m2) and saturation temperature (8–12°C). Data are compared with smooth tube counterpart. It was found that both heat transfer coefficient and pressure drop increased as mass flux increased. The range of pressure drop penalty factor (1.10–1.70) was slightly smaller than that of heat transfer enhancement factor (1.39–1.79). Data are compared with available heat transfer and pressure drop correlations.

Application of the Natural Refrigerant Mixture R-290/DME to a Soft Ice Cream Refrigerator

Due to the ozone depletion issue, R-502, which had long been used as the refrigerant of an ice cream refrigerator, has been replaced by R-404A. However, global warming potential (GWP) of R-404A is high, and thus, a replacement refrigerant is necessary in the long term. Natural refrigerants, such as R-290 or DME (dimethylether), could be a choice. In this study, an ice cream refrigerator cycle was optimized using R-290/DME mixture (mass fraction 65/35). The optimization was accomplished through a search for the proper refrigerant charge amount and the opening of the expansion valve. For the present ice cream refrigerator having 2.8L freezer volume, the optimum charge amount was 900g, and the optimum valve opening was +120∘. At this configuration, the ice cream formation time was 3′ 6′′ and COP was 2.0. The ice cream formation time was much shorter than when R-404A was used, and the COP was increased by more than 100%. For actual usage of the refrigerant, however, the flammability issue of the R-290/DME mixture should be cleared.

Hydrodynamic and heat transfer characteristics of glass bead-water flow in a vertical tube

Abstract Pressure loss and heat transfer characteristics of the glass bead—water flow were investigated. Three different sizes (dp = 1.5 mm, 3.0 mm, 4.0 mm) of particles having 2.54 specific gravity were tested. The particles augmented the heat transfer as well as increased the pressure loss. The enhancement increased as the particle volume fraction increased and as the flow velocity decreased. Through the flow visualization study, the heat transfer and pressure loss characteristics of the slurry flow are shown to be closely related with the particle behavior near the wall, i.e., wall hitting frequency, particle slip velocity, etc.