Horizontal Small Diameter Tube Research Articles

ヘリウムガスを用いた水平細管内強制対流定常熱伝達に関する数値解析

The numerical simulation of turbulent heat transfer for helium gas flowing in a horizontal small-diameter tube was performed. The numerical simulation conditions were 0.8 mm and 1.8 mm for inner diameters, and 102-128 m/s and 87-241 m/s for velocities at the inlet of the tube, respectively. The numerical solution was carried out by applying ANSYS FLUENT software. The standard k-ε model and a user-defined wall function were used for the analysis. The thickness of the conductive sublayer, which is the region where heat transfer by conduction is dominant, was estimated. As a result of the numerical simulation, it was found that the thickness of the conductive sublayer becomes thinner as the flow velocity increases and the inner diameter decreases. Moreover, the heat transfer coefficients of the simulation data were compared with the previous experimental data, and they were in good agreement with the experimental data.

Two-phase pressure drop of ammonia in horizontal small diameter tubes: Experiments and correlation

A series of experiments are conducted to investigate the two-phase pressure drop of ammonia in horizontal small diameter tube. The experiments are performed at saturation temperatures of −15.8–4.6 °C, and the mass flux range is from 20 to 100 kg m−2 s−1. This paper analyzes the influence mechanisms of vapor quality, mass flux, saturation temperature, and tube diameter on the two-phase frictional pressure gradient in detail. Meanwhile, several correlations are compared with all the experimental data and the results show that the Müller-Steinhagen and Heck (1986) correlation gives the best agreement. With the analysis of the major forces for ammonia two-phase flow under the tested experimental conditions, the Reynolds number and Bond number are introduced to modify the Müller-Steinhagen and Heck (1986) correlation. Lastly, a modified correlation is developed with the mean absolute deviation of only 13.5%, and 89.2% of the data are within the ± 30% error band. In addition, the modified correlation is evaluated based on a complied database which contains 951 experimental data points from 11 published literatures.

Heat transfer and pressure drop characteristics of ammonia during flow boiling inside a horizontal small diameter tube

In this article, a series of experiments was conducted to investigate the flow boiling heat transfer and pressure drop characteristics of ammonia in a 4 mm horizontal plain tube. The experiments were performed for heat fluxes at 9 and 21 kW m−2 K−1, mass flux from 50 to 100 kg m−2 s−1, and saturation temperature from −15.8 to 5 °C. The experimental results show that with the increase of heat flux, the heat transfer coefficient increases. Meanwhile, it also increases with a rise in mass flux in the low vapor quality region, whereas reversed situation can take place in the high vapor quality region when the saturation temperature is decreased. The saturation temperature has little effect on the heat transfer coefficient in the low vapor quality region, however at higher mass fluxes, the heat transfer coefficient can decrease with decreasing saturation temperature in the high vapor quality region. The comparisons of the experimental data with existing correlations for flow boiling heat transfer coefficient show that Gungor and Winterton correlation can give good agreement with mean absolute deviation of 19.6% even though it is developed for turbulent flow. The adiabatic two-phase frictional pressure gradient increases with the increase of vapor quality and mass flux, while decreases with the increase of saturation temperature. The comparisons with existing correlations for two-phase frictional pressure gradient indicate that Müller-Steinhagen and Heck correlation can predict the experimental data well with mean absolute deviation of 16.1% and 93.4% of the data is within the ±30% error band.

Flow boiling instabilities of nitrogen in horizontal small diameter tube

The flow boiling instability characteristics of nitrogen flow boiling in a horizontal small diameter tube were numerically studied, and a numerical model coupled with two phase flow boiling and system flow supplying was developed. With the steady state solution of the model, the flow boiling characteristic curve and flow supplying curve were obtained. The flow boiling characteristic curve was affected by the heat flux and inlet subcooling, meanwhile the flow supplying curve was affected by the total system pressure drop and friction feature of the flow supplying section. With the transient solution of the model, The dynamical response of pressure drop oscillations type (PDOs) instability of two phase flow were investigated, occurring region and trigging conditions were also obtained.

Effect of tube diameter on boiling heat transfer and flow characteristic of refrigerant R32 in horizontal small-diameter tubes

This study investigated the effect of tube diameter on flow boiling characteristics of refrigerant R32 in horizontal small-diameter tubes with 1.0, 2.2, and 3.5 mm inner diameters. The boiling heat transfer coefficient and pressure drop were measured at 15 °C saturation temperature. The effects of mass velocity, heat flux, quality, and tube diameter were clarified. The flow pattern of R32 for adiabatic two-phase flow in a horizontal glass tube with an inner diameter of 3.5 mm at saturation temperature of 15 °C was investigated. Flow patterns such as plug, wavy, churn, and annular flows were observed. The heat transfer mechanisms of forced convection and nucleate boiling were similar to those in conventional-diameter tubes. In addition, evaporation heat transfer through a thin liquid film in the plug flow region for low quality, mass velocity, and heat flux was observed. The heat transfer coefficient increased with decreasing tube diameter under the same experimental condition. The fictional pressure drop increased with increasing mass velocity and quality and decreasing tube diameter. The experimental values of the heat transfer coefficient and frictional pressure drop were compared with the values calculated by the empirical correlations in the open literature.

Characteristics of heat transfer for CO2 flow boiling at low temperature in mini-channel

This paper presents an experimental and theoretical investigation on two-phase flow boiling heat transfer with carbon dioxide (CO2 or R744) as the refrigerant in horizontal small diameter tube at low temperature. The experimental data were obtained in the following condition, the heat flux is 7.5–30kW/m2; the mass flow rate is 300–600kg/m2s; the saturation temperature is 233–273K; the test section is made of stainless steel tubes whose inner diameters are 0.6 and 1.5mm. The experimental results show that heat transfer coefficient increases with the increase of mass flow rate, but decreases with the increase of tubes’ inner diameter and saturation temperature, the boiling heat transfer of CO2 has a greater effect on nucleate boiling and highly depends on heat flux. The comparative study indicates that: during the existed flow boiling heat transfer model, the model which has the best prediction accuracy presented with 79.8% of predicted points within ±30% and 21.8% mean deviation before the dryout phenomenon happened, while it was only 18.4% and 59.9% after the dryout phenomenon happened.

Effect of tube diameter on boiling heat transfer of R-134a in horizontal small-diameter tubes

The boiling heat transfer of refrigerant R-134a flow in horizontal small-diameter tubes with inner diameter of 0.51, 1.12, and 3.1 mm was experimentally investigated. Local heat transfer coefficient and pressure drop were measured for a heat flux ranging from 5 to 39 kW/m 2, mass flux from 150 to 450 kg/m 2 s, evaporating temperature from 278.15 to 288.15 K, and inlet vapor quality from 0 to 0.2. Flow patterns were observed by using a high-speed video camera through a sight glass at the entrance of an evaporator. Results showed that with decreasing tube diameter, the local heat transfer coefficient starts decreasing at lower vapor quality. Although the effect of mass flux on the local heat transfer coefficient decreased with decreasing tube diameter, the effect of heat flux was strong in all three tubes. The measured pressure drop for the 3.1-mm-ID tube agreed well with that predicted by the Lockhart–Martinelli correlation, but when the inner tube diameter was 0.51 mm, the measured pressure drop agreed well with that predicted by the homogenous pressure drop model. With decreasing tube diameter, the flow inside a tube approached homogeneous flow. The contribution of forced convective evaporation to the boiling heat transfer decreases with decreasing the inner tube diameter.

10512 細管内での沸騰熱伝達特性に関する研究(熱工学・乱流)

Two-phase flow boiling heat transfer of water in a horizontal small diameter tube, ID=0.9mm, was experimentally examined at a atmospheric pressure condition in a range of mass flux from 84 to 255kg/m^2s. The correlation developed for a 1.45mm ID tube was applied to predict pressure drop and local heat transfer coefficient. Predicted pressure drop was within an error of 20%, at a higher heat flux, however, the error increased to be up to 40%. The effect of mass flux and location on the heat transfer coefficient was small. The heat transfer correlation well predicted at a range of Re_G<2000, but it gave worse predictions at Re_G>2000.

水平細管内の気液二相流動(流体工学(混相流II))

水平細管内の気液二相流動(流体工学(混相流II))

Critical quality prediction for saturated flow boiling of CO 2 in horizontal small diameter tubes

The dryout for flow boiling carbon dioxide (CO 2) in horizontal small diameter tubes is investigated through experiment and theoretical modeling. Tests are conducted in conditions where the saturation temperature is 0, 5, and 10 °C, heat flux is 7.2–48.1 kW/m 2 and mass flux is 500–3000 kg/m 2 s. The dryout phenomena of CO 2 are similar with those of water in many respects, while the effects of mass flux on dryout show differences among them. The dryout of CO 2 is predicted by a theoretical dryout model, which is developed and verified with steam–water data. Two entrainment mechanisms of interface deformation and bubble bursting are considered in the model and dryout is determined when the liquid film thickness is less than the critical liquid film thickness, the criteria film thickness of dryout. The present model well predicts the experimental critical qualities except when mass flux is relatively high, at which the deposition of liquid droplet on the liquid film and the occurrence of dryout patches become very significant.