Tailored flow condensation of low surface tension fluids via additively manufactured gradient wick structures

Condensation flow patterns and heat transfer in horizontal microchannels

The greater part of the horizontal condenser tube is occupied by the stratified and annular flows, which play important roles in condensation heat transfer coefficients. Both a volume of fluid (VOF) interface tracking method and k-ε two equations model was applied to analyze the characteristics of the stratified and annular flows for horizontal tubes, obtaining distribution of velocity, contours of both temperature and local condensation heat transfer coefficients. The computation shows that an annular flow having thinner liquid film attached on the inner surface of tubes appears at the inlet of the horizontal condenser tube because vapor exerts a higher interfacial shear stress on the gas-liquid interface, therefore, there are a higher local condensation heat transfer coefficients there. Next, as vapor quality decreases and condensate gravity increases along the condenser tube length, the condensation flow pattern transforms from the annular flow into the stratified flow in which local condensation heat transfer coefficients decreases and distributes unevenly along the circumference as liquid film at the bottom of the condenser tube become thicker. In addition, in the stratified flow, a wave structure is formed in the middle of the condensate pool at the bottom of the condenser tube because condensate on the top of the condenser tube slides into the bottom of the condenser tube and collapse each other. The heat transfer rate calculated by the present method is compared to those predicted from a Shah correlation, the agreement is found to be good, and both errors is within 7%.

By using unique experimental techniques and the careful construction of an experimental apparatus, the characteristics of the local heat transfer were investigated using the condensing R134a two-phase flow in horizontal single mini-channels. The circular channels (D h = 0.493, 0.691, and 1.067 mm) and rectangular channels (Aspect Ratio = 1.0; D h = 0.494, 0.658, and 0.972 mm) were tested and compared. Tests were performed for a mass flux of 100, 200, 400, and 600 kg/m2s, a heat flux of 5 to 20 kW/m2, and a saturation temperature of 40°C. In this study, the effect of heat flux, mass flux, vapor qualities, hydraulic diameter, and channel geometry on flow condensation were investigated, and the experimental local condensation heat transfer coefficients are shown. The experimental data of condensation Nusselt number are compared with existing correlations.

By using unique experimental techniques and careful construction of the experimental apparatus, the characteristics of the local heat transfer were investigated using the condensing R134a two-phase flow, in horizontal single mini-channels. The circular channels (Dh = 0.493, 0.691, and 1.067 mm) and rectangular channels (Dh = 0.494, 0.658, and 0.972 mm) were tested and compared. Tests were performed for a mass flux of 100, 200, 400, and 600 kg/m2s, a heat flux of 5 to 20 kW/m2, and a saturation temperature of 40°C. In this study, effect of heat flux, mass flux, vapor qualities, hydraulic diameter, and channel geometry on flow condensation were investigated and the experimental local condensation heat transfer coefficients are shown. The experimental data of condensation Nusselt number are compared with existing correlations.

The characteristics of local heat transfer and pressure drops were experimentally investigated using condensing R134a two-phase flow, in single rectangular tubes, with hydraulic diameter of 0.494, 0.658, and 0.972 mm. New experimental techniques were used to measure the in-tube condensation heat transfer coefficient especially for the low heat and mass flows. Tests were performed for a mass flux of 100, 200, 400, and 600 kg/m2s, a heat flux of 5 to 20 kW/m2, and a saturation temperature of 40°C. In this study, effect of heat flux, mass flux, vapor qualities, and hydraulic diameter on flow condensation were investigated and the experimental local condensation heat transfer coefficients and frictional pressure drop are shown. The experimental data of condensation Nusselt number are compared with previous correlations, most of which are proposed for the condensation of pure refrigerant in a relatively large inner diameter round tubes.

Plate heat exchangers are widely used for two-phase heat transfer in the industrial applications, and recently more attention has been paid to the plate heat exchangers with enhanced surface due to their better heat transfer performance. In this paper, the local condensation heat transfer coefficients are studied using R134a in a micro-structured plate heat exchanger. In order to obtain a more accurate prediction model, a series of measurements are conducted under various operating conditions. The mass flux of R134a varied from 47 kg/m2s to 77 kg/m2s, the saturation pressure in the condenser ranged from 6.32 bar to 8.95 bar, and the value of the heat flux was between 13 kW/m2 and 22 kW/m2. The local two-phase Nusselt number increases with the increase of the mass flux. As the saturation pressure increases, the local two-phase Nusselt number increase at the beginning of the condensation and decrease at the end of the condensation. However, the effect of heat flux on local heat transfer is irregular, due to the interaction of these parameters in the experiment. Comparing with the unstructured plate heat exchanger, R134a condenses faster at the beginning of the process in the micro-sturctured plate heat exchanger, and the local heat transfer performs better when the vapor quality is lower. Combing with the phenomenon that the overall heat flux in micro-structured plate is larger under the same working conditions, it shows that the overall heat transfer of the micro-structured plate is improved, but the local heat transfer uprades only at lower vapor qualities. A new correlation is developed, it predicts all the experimental data within the root mean square error 10%, and a new correlation for the waterside is suggested as well.

A numerical study on condensation flow and heat transfer of refrigerant in minichannels of printed circuit heat exchanger

Experimental and numerical investigations of local condensation heat transfer in a single square microchannel under variable heat flux

This paper reviews recent Korean studies of flow characteristics, flow boiling, and flow condensation in micro- and mini-channels. The characteristics of local heat transfer and pressure drops were experimentally investigated using condensing R134a two-phase flow, in a single round tube, with an inner diameter of 0.691 mm. New experimental techniques were developed to measure the condensation heat transfer coefficient. Tests were performed for a mass flux of 100 to 600 kg/m2s, a heat flux of 5 to 20 kW/m2, and a saturation temperature of 40°C. The experimental local condensation heat transfer coefficients and two-phase frictional pressure gradients are shown. Comparisons of experimental data with existing models reveal that the correlations failed to predict the present data. This study contains the unique sub-millimeter-diameter, single round tube, condensation data reported in the literature.

Ammonia condensation in the horizontal and vertical straight inner-grooved tubes and annuli

Numerical simulation on the condensation heat transfer and flow characteristics outside smooth and enhanced tubes

A tubular condenser column was designed and constructed to evaluate the performances of the condensation process with a high fractional noncondensable gas in the dewvaporation desalination process. An experimental investigation has been conducted to determine the local condensation heat transfer coefficient, which is related to steam−air mixed gas condensation in an annulus around a vertical copper tube. The results show that the local heat transfer coefficient decreases with increase of the distance from the mixed gas inlet, and varies from 200 to 1900 W/m2·°C, which is high enough for steam condensation in the presence of a high fractional noncondensable gas. The correlation of steam condensing local heat transfer coefficient in the presence of a high fractional noncondensable gas was developed in terms of the local mixture Reynolds number, Jakob number, and ratio of noncondensable gas mass fraction. It is found that the correlation has a good agreement with the experimental data.

A tubular condenser column was designed and constructed to evaluate the performances of the condensation process with a high fractional noncondensable gas in the dewvaporation desalination process. An experimental investigation has been conducted to determine the local condensation heat transfer coefficient, which is related to steam−air mixed gas condensation in an annulus around a vertical copper tube. The results show that the local heat transfer coefficient decreases with increase of the distance from the mixed gas inlet, and varies from 200 to 1900 W/m2·°C, which is high enough for steam condensation in the presence of a high fractional noncondensable gas. The correlation of steam condensing local heat transfer coefficient in the presence of a high fractional noncondensable gas was developed in terms of the local mixture Reynolds number, Jakob number, and ratio of noncondensable gas mass fraction. It is found that the correlation has a good agreement with the experimental data.

Flow condensation in horizontal tubes

Local, on-tube condensation heat transfer coefficients of R134a alternatives, R450A and R513A, were measured on a single, horizontal, smooth 15.875-mm-diameter stainless-steel tube at saturation temperatures of 35 °C and 40 °C in a quiescent vapor space for a range of subcoolings (i.e., 1.7 °C–7.2 °C) and heat fluxes (i.e., 5–16 kW/m2). Gravity-driven flows were visualized through site glasses and droplet-mode flow was observed. Experiments were validated using Nusselt’s correlation for on-tube condensation. Local heat fluxes were used to compute local on-tube condensation heat transfer coefficients and the wall temperature was directly measured. Data were presented for two radial positions (i.e., angles β = 11° and β = 109° from the vertical). For both refrigerants, local heat transfer coefficients at β = 11° were higher than at β = 109° due to the thinner condensed film. At the same subcooling and at a saturation temperature of 35 °C and 40 °C, local heat transfer coefficients for R513A were higher than R450A. Local heat transfer coefficients were also compared between new (i.e., 3–6 months) and stored (i.e., 3-year old) R513A and R450A, and there were minimal differences. A gas chromatography analysis, and analysis using the vapor liquid equilibrium assumption, showed that there were modest composition changes for either one or both stored refrigerant components.