Investigation of Heat Transfer and Pressure Drop for R744 in a Horizontal Smooth Tube of R744/R404A Hybrid Cascade Refrigeration System—Part 1: Intermediate Temperature Region

Abstract

In this study, the evaporation heat transfer characteristics of an intermediate temperature of R744 in a smooth horizontal tube, when operating as an indirect refrigeration system (IRS) among hybrid cascade refrigeration systems (HCRSs), are evaluated. Studies on the characteristics of intermediate-temperature evaporation heat transfer under the operating conditions of evaporators used in actual refrigeration systems, such as IRS, cascade refrigeration systems, and HCRS, used in supermarkets are lacking. Thus, this study provides basic data on the characteristics of evaporation heat transfer of R744 in the evaporators of refrigerators used in supermarkets. The tube employed to the evaporation experiment in this study was a horizontal smooth copper tube with a length of 8000 mm and an inner diameter of 11.46 mm. The experimental variables were measured over a wide range of mass flux of 200–500 kg/(m2·s), heat flux of 10–40 kW/m2, and saturation temperature of −40–0 °C. The main results are summarized as follows: (1) The application of Kandlikar’s correlation formula at an evaporation temperature of −20 °C in an IRS helps in a good prediction of the R744 evaporation heat transfer coefficient. (2) The pressure drop according to the heat and mass flux showed the same heat transfer coefficient trend, but the pressure drop at saturation temperature was different from the trend of heat transfer coefficient.

Investigation of Heat Transfer and Pressure Drop for R744 in a Horizontal Smooth Tube of R744/R404A Hybrid Cascade Refrigeration System—Part 2: Low-Temperature Region

“…By contrast, the correlation formulas of Gungor-Winterton [42], Kenning-Cooper [44], and Yoon et al [45] have large deviation rates, which is consistent with the results presented in Jeon [23]. The correlation formula of Gungor-Winterton [42] uses experimental constants based on experimental data, and it has a high deviation rate because the experimental range is out of the application range of the correlation formula.…”

“…A detailed schematic diagram of the R744 evaporator used herein is presented in Figure 2, and its specifications are listed in Table 1. Descriptions of the experiment, measurement method, heat transfer & pressure drop process and data reduction procedure can be found in a previous paper [23].…”

“…In this section, as in Jeon [23], comparative correlation formulas (Chen [41], Gungor and Winterton [42], Kandlikar [43], Kenning-Cooper [44], and Yoon et al [45]) are used to calculate the average deviation, and absolute mean deviation is applied to compare these correlations. The related correlation formulas are presented in Table 8.…”

“…In the correlation formula of Kandlikar [43], Ffl is replaced by 1, as in Jeon [23]. This is because Ffl = 1 for all refrigerants, except those studied by Kandlikar [46] and Kandlikar and Balasubramanian [47].…”

“…Thus, it can be concluded that heat flux does not have a significant effect on the frictional pressure drop of the two-phase fluid. The pressure drop measured under the experimental conditions is the sum of the pressure drop due to friction (∆P 𝑓𝑟𝑖𝑐𝑡 ) and that due to momentum (∆P 𝑚𝑜𝑚 ), as shown in Jeon [23]. Because the pressure drop due to momentum in a short section is negligible, the experimental pressure drop in this paper refers only to the frictional pressure drop.…”