Evaporation Heat Transfer and Pressure Drop of R32 inside Small-diameter 4.0 mm Tubes

Experiments were conducted examining pressure drop and heat transfer for flow boiling inside horizontal smooth and two internally helical-grooved small-diameter tubes with outer diameter of 4.0 mm. The geometric parameters of internally helical-grooved tubes are the groove number of 25 and 40, helix angle of 16 and 17 °, fin height of 0.1 and 0.2 mm. The heat transfer and pressure drop are measured in a range of mass velocities from 50 to 400 kg/(m2s) and a range of heat fluxes from 5 to 20 kW/m2 at saturation temperature 15 °C. The pressure drop of microfin tubes were 1.1 - 2.0 times than the smooth tube. The heat transfer coefficient of microfin tubes were 1.3 - 6.5 times than the smooth tube. The pressure drop and heat transfer characteristics of the internally helical-grooved tubes for evaporation flow were examined and compared with those of the smooth tube

Effect of Fin Geometries on Condensation Heat Transfer and Pressure Drop inside Horizontal Small-Diameter 4 mm Microfin Tubes

In the refrigeration and air-conditioning fields, small-diameter microfin tubes with an outside diameter of 5 mm or less have been developed; these tubes enhance energy savings by improving the performance of the heat exchanger and by reducing the charge amount of the refrigerant for downsizing the heat exchanger. Small-diameter microfin tubes with 4 mm outer diameters have been utilized as a part of practical applications. It is important to investigate heat transfer and pressure drop characteristics at lower mass velocities. Furthermore, hydrofluoroolefin (HFO) refrigerants have been recently attracting attention as low global warming potential (GWP) refrigerants. However, only limited studies exist on condensation heat transfer of HFO refrigerants in small-diameter microfin tubes. This study experimentally investigated the condensation heat transfer and pressure drop characteristics of R1234ze(E) inside horizontal small-diameter 4.0 mm OD microfin tubes having three different types of fin geometries. The specifications of the three fin geometries were 40 fins with a fin height of 0.18 mm and a helix angle of 18°, 50 fins with a fin height of 0.15 mm and a helix angle of 12°, and 50 fins with a fin height of 0.12 mm and a helix angle of 25°. The experiments were carried out for a range of mass velocities, from 50 to 400 kgm-2s-1, and at a saturation temperature of 35 °C. The effects of fin geometries such as the number of fins, fin height, and helix angle on the heat transfer and pressure drop were investigated. The heat transfer coefficient increased as the number of fins increased for the lowest mass velocity. Fin height was most effective on heat transfer enhancement at higher mass velocities. The heat transfer coefficient and pressure drop of the microfin tubes were compared with those of smooth tubes and were evaluated in terms of the enhancement ratio of heat transfer and the increment ratio of pressure drop. The measured heat transfer coefficient and pressure drop were compared with previous correlations for small-diameter microfin tubes

Boiling Heat Transfer and Pressure Drop of R1234ze(E) inside a Small-Diameter 2.5 mm Microfin Tube

Currently, the development of high-performance and compact heat exchangers with small- diameter tubes having a hydraulic diameter of less than 5 mm is needed in order to improve the performance of the heat exchanger and to reduce the refrigerant charge for air-conditioning systems. The effects of surface tension and shear stress on boiling heat transfer and flow characteristics become dominant as the tube diameter decreases. In addition, these effects are different from those in conventional-diameter tubes. It is necessary to clarify boiling heat transfer and pressure drop to facilitate the design of evaporators. Furthermore, low global warming potential refrigerants such as HFOs have been attracting attention. However, only limited research is available on the boiling heat transfer and pressure drop of HFO refrigerants in small-diameter microfin tubes. This study experimentally investigated the boiling heat transfer and pressure drop of R1234ze(E) in a horizontal small-diameter microfin tube having 2.5 mm outer diameter and 2.1 mm equivalent diameter. The boiling heat transfer and pressure drop were measured in a mass velocity range of 100–400 kg/(m2s) and heat flux range of 5–20 kW/m2 at a saturation temperature of 15°C. The boiling heat transfer coefficient at the mass velocity of 200 kg/(m2s) exhibited the highest value at the dominant region of tin liquid film evaporation heat transfer. The measured boiling heat transfer coefficient agreed well with previous correlations in only the dominant region of forced convection evaporation. The frictional pressure drop increased with increasing mass velocity and vapor quality. The measured pressure drop agreed well with previous correlations for conventional-diameter microfin tubes

Correlation for Flow Boiling Heat Transfer of Low-pressure Refrigerants Inside A Horizontal Smooth Tube

The flow boiling heat transfer of low-pressure refrigerants including R1224yd(Z), R1233zd(E), R1336mzz(Z), and R1336mzz(E) was experimentally investigated inside a horizontal smooth tube with outer and inner diameters of 9.52 and 8.40 mm, respectively. The experiments were conducted at a mass velocity range of 50–300 kgm−2s−1 and a saturation temperature of 40 °C. In the forced convection region, where the vapor shear force was dominant, the measured heat transfer coefficients were slightly larger than those of the conventional refrigerants were. A modified heat transfer correlation was developed based on a wettability separation angle to differentiate the stratified and annular flows. In addition, the heat transfer coefficient after the onset of dryout vapor quality was studied. Within the entire database, the mean deviation of the modified correlation was 1.0 %. The developed correlation agreed better with the experimental results compared to the conventional correlations

Flow Boiling Heat Transfer Characteristics of R32 inside a Horizontal Small-diameter Microfin Tube

Currently, the development of high-performance and compact heat exchangers using small diameter tubes of hydraulic diameter less than 5 mm is required to improve the performance of heat exchanger and reduce the charge amount of refrigerant for air-conditioning systems. For microfin tubes, the tube diameter become smaller, however, the effects of surface tension and shear stress on boiling heat transfer and flow characteristics in small diameter tubes are different from those in conventional large diameter tubes. This study experimentally investigates the flow boiling heat transfer and pressure drop characteristics of low global warming potential refrigerant R32 in horizontal small-diameter microfin tube with 3.0 mm outer diameter and 2.6 mm mean diameter. The flow boiling heat transfer and pressure drop characteristics are measured in a range of mass velocities from 50 to 400 kg/(m2 s) and a range of heat fluxes from 5 to 20 kW/m2 at saturation temperature of 15°C. The flow boiling heat transfer characteristics were compared with those of the 4 mm microfin tube

Robust estimation of bacterial cell count from optical density

Optical density (OD) is widely used to estimate the density of cells in liquid culture, but cannot be compared between instruments without a standardized calibration protocol and is challenging to relate to actual cell count. We address this with an interlaboratory study comparing three simple, low-cost, and highly accessible OD calibration protocols across 244 laboratories, applied to eight strains of constitutive GFP-expressing E. coli. Based on our results, we recommend calibrating OD to estimated cell count using serial dilution of silica microspheres, which produces highly precise calibration (95.5% of residuals <1.2-fold), is easily assessed for quality control, also assesses instrument effective linear range, and can be combined with fluorescence calibration to obtain units of Molecules of Equivalent Fluorescein (MEFL) per cell, allowing direct comparison and data fusion with flow cytometry measurements: in our study, fluorescence per cell measurements showed only a 1.07-fold mean difference between plate reader and flow cytometry data

Evaporation heat transfer and flow characteristics of vertical upward flow in a plate-fin heat exchangerTransfert de chaleur par évaporation et caractéristiques de l\u27écoulement vertical ascendant dans un échangeur de chaleur à plaque à ailettes

This study experimentally investigated the evaporation heat transfer and two-phase flow characteristics of a vertical upward flow using R1234ze(E) and R32 as test refrigerants in a plate-fin heat exchanger with offset fins. The adiabatic and evaporation flows of R1234ze(E) were visualized using a high-speed camera.Owing to the asymmetry of the inlet and outlet of the channel, a non-uniform velocity profile in the channel-width direction was observed under both adiabatic and evaporation flows.Evaporation flow accompanied by nucleate bubble generation at the lower area of the channel and the occurrence of a dry area at the upper side of the channel were observed.The heat transfer coefficients of R1234ze(E) and R32 were measured, and the effects of mass flux, quality, and heat flux on heat transfer were clarified. The heat transfer was strongly affected by forced convection at higher mass fluxes and quality whereas nucleate boiling dominated at higher heat fluxes and lower quality. The difference in the heat transfer characteristics at local positions in the channel was also investigated by measuring the local wall temperatures.Comparing refrigerants, the heat transfer coefficient of R32 was higher than that of R1234ze(E), regardless of the mass flux and quality, especially at lower mass flux conditions.公開日: 2023-12-0

Correlation for Condensation Heat Transfer in a 4.0 mm Smooth Tube and Relationship with R1234ze(E), R404A, and R290

In this study, the condensation heat transfer coefficient and pressure drop characteristics of a 4 mm outside diameter smooth tube, using R32, R152a, R410A, and R1234ze(E) refrigerants, were examined. Condensation heat transfer coefficients and pressure drops were measured at a saturation temperature of 35 °C, in the region of mass velocities from 100 to 400 kg m−2s−1. The frictional pressure drop, and the condensation heat transfer from the new measurements, using R1234ze(E) as a refrigerant, were compared with those of R32, R152a, and R410A, in the smooth tube. Experimental values of condensation heat transfer coefficient of smooth tube were also compared to the predicted values obtained using the previously established correlations. The previous correlation from Cavallini et al., for the condensation heat transfer coefficient of small-diameter smooth tube, was estimated to be within ±30%. However, the general correlation, which can be easily predicted, for condensation heat transfer inside small-diameter smooth tubes, was suggested, and the relationship of the general correlation was compared with data for R1234ze(E) obtained by us, and R404A and R290 obtained by other researchers