Condensation in a vertical tube bundle passive condenser – Part 1: Through flow condensation

An experimental study and a boundary layer analysis were performed for the steam condensation in a vertical tube bundle passive condenser operating in a through flow mode. Four condenser tubes were submerged in a water pool and the heat from the condenser tube was removed through boiling. Experimental data were obtained for various system pressures (100–170 kPa), inlet steam flow rates (15–47 g/s) and non-condensable gas concentration (0–15%). The experimental results showed substantial deterioration in condensation when non-condensable gas was present. With increase in steam flow rate and system pressure the condensate rate increased. The boundary layer thickness and non-condensable gas concentration increased along the condenser tube length.     

Steam condensing – liquid CO2 boiling heat transfer in a steam condenser for a new heat recovery system

In a new waste heat recovery system, waste heat is recovered from steam condensers through cooling by liquid CO₂ instead of seawater, taking advantage of effective boiling heat transfer performance; the heat is subsequently used for local heat supply. The steam condensing – liquid CO₂ boiling heat transfer performance in a steam condenser with a shell and a helical coil non-fin tube was studied both numerically and experimentally. A heat transfer numerical model was constructed from two models developed for steam condensation and for liquid CO₂ boiling. Experiments were performed to verify the model at a steam pressure range of 3.2–5 kPa and a CO₂ saturation pressure range of 5–6 MPa. Overall heat transfer coefficients obtained from the numerical model agree with the experimental data within ±5%. The numerical estimations show that the boiling local heat transfer coefficient reaches a maximum value of 26 kW/m² K. This value is almost one order higher than that of a conventional water-cooled condenser.     

Transient and steady-state natural convection heat transfer from a heated horizontal concrete cylinder

In refrigeration systems, it is possible to reduce energy consumption (compressor power) and increase COP by decreasing the condensation temperature. Decreasing the condensation temperature can be achieved either by increasing the overall heat transfer coefficient or heat transfer surface area of the condenser. Usually, the radiuses of condenser tubes of domestic refrigerators are quite smaller than the critical radius. Thus, the radius can be increased up to the critical radius by coating the bare condenser tube to increase heat transfer. On the other hand, refrigerators operate discontinuously depending on the ambient temperatures. Coating material stores some of the heat during the working period and continues heat transfer during the off period so that the condenser continues transferring heat while the compressor is not working. Storage effect depends on the specific heat and density of the coating material. Transient and steady-state natural convection heat transfer from a heated horizontal cylinder covered with concrete layer by molding is studied experimentally and numerically to determine the effects of the parameters considered above. The copper and the concrete test cylinders used in the experimental study have a length of 1 m and outer diameter of 9.45 mm and 68.5 mm respectively. The ambient and copper cylinder surface temperatures varied between 20 °C÷30 °C and 30 °C÷50 °C respectively. Constant heat flux was applied to bare and concrete cylinders. Transient heat transfer experiments were performed when bare, and concrete cylinders were reached to steady state condition. Heat transfer rates under transient conditions from bare and concrete horizontal cylinders were compared and heat transfer enhancement was determined. Based on the experimental data average Nusselt numbers were calculated and compared with the well known correlations. Also temperature distributions obtained from numerical simulations were very close to the experimental data. The effect of the decrease in the temperature of the inner copper cylinder surface (condensation temperature) on COP was investigated considering an ideal Carnot refrigeration cycle. It is found that the enhancement in COP of a Carnot refrigeration cycle is 35.7% under transient condition.     

Optimum temperatures in a shell and tube condenser with respect to exergy

This paper focuses on evaluation of the optimum cooling water temperature during condensation of saturated water vapor within a shell and tube condenser, through minimization of exergy destruction. First, the relevant exergy destruction is mathematically derived and expressed as a function of operating temperatures and mass flow rates of both vapor and coolant. The optimization problem is defined subject to condensation of the entire vapor mass flow and it is solved based on the sequential quadratic programming (SQP) method. The optimization results are obtained at two different condensation temperatures of 46 °C and 54 °C for an industrial condenser. As the upstream steam mass flow rates increase, the optimal inlet cooling water temperature and exergy efficiency decrease, whereas exergy destruction increases. However, the results are higher for optimum values at a condensation temperature of 54 °C, compared to those when the condensation temperature is 46 °C. For example, when the steam mass flow rate is 1 kg/s and the condensation temperature increases from 46 °C to 54 °C, the optimal upstream coolant temperature increases from 16.78 °C to 25.17 °C. Also, assuming an ambient temperature of 15 °C, the exergy destruction decreases from 172.5 kW to 164.6 kW. A linear dependence of exergy efficiency on dimensionless temperature is described in terms of the ratio of the temperature difference between the inlet cooling water and the environment, to the temperature difference between condensation and environment.     

Effect of cross sectional shape during condensation in a single square minichannel

The objective of this work is to present new condensation heat transfer coefficients measured inside a single square cross section minichannel, having a 1.18 mm side length, and compare them to the ones previously measured in a circular minichannel. Tests have been performed with R134a at 40 °C saturation temperature, at mass velocity ranging between 200 and 800 kg m^?2 s^?1. As compared to the heat transfer coefficients measured in the circular cross section channel, for the same hydraulic diameter, in the square minichannel the authors find a heat transfer enhancement at the lowest value of mass velocity, which must be due to the effect of surface tension. No heat transfer coefficient increase has been found at the highest values of the mass velocity where condensation is shear stress dominated.     

Air heat exchangers with long heat pipes: Experiments and predictions

This paper presents measurements and predictions of a heat pipe-equipped heat exchanger with two filling ratios of R134a, 19% and 59%. The length of the heat pipe, or rather thermosyphon, is long (1.5 m) as compared to its diameter (16 mm). The airflow rate varied from 0.4 to 2.0 kg/s. The temperatures at the evaporator side of the heat pipe varied from 40 to 70 °C and at the condenser part from 20 to 50 °C. The measured performance of the heat pipe has been compared with predictions of two pool boiling models and two filmwise condensation models. A good agreement is found. This study demonstrates that a heat pipe equipped heat exchanger is a good alternative for air–air exchangers in process conditions when air–water cooling is impossible, typically in warmer countries.     

Determination of annular condensation heat transfer coefficient of steam in microchannels with trapezoidal cross sections

Experiments have been carried out to determine annular condensation heat transfer coefficient of steam in two silicon microchannels having trapezoidal cross sections with the same aspect ratio of 3.15 at 54 < G < 559 kg/m² s under 3-side cooling conditions. A semi-analytical method, based on turbulent flow boundary layer theory of liquid film with correlations of pressure drop and void fraction valid for microchannels, is used to derive the annular local condensation heat transfer coefficients. The predicted values based on the semi-analytical model are found within ±20% of 423 data points. It is shown that the annular condensation heat transfer coefficient in a microchannel increases with mass flux and quality and decreases with the hydraulic diameter.     

Study on flow characteristics and pressure distribution along a heated channel in subcooled flow boiling

In the present work, an experimental investigation is presented which is focused on the pressure drop along the heated as well as the unheated zone of a flow channel. The experiments were performed by using a 16 mm stainless steel ID tube with a 350 mm heated length followed by a 500 mm unheated extension downstream. Pressure drops and pressure distribution were measured along the flow channel, with water, at mass flow velocities of 6342–9513 kg/m² s and at heat fluxes of 3.37–4.1 MW/m². The whole experiments were conducted near atmospheric pressure by keeping the outlet of the flow channel open to the atmospheric pressure. The experimental results show a reduction of the pressure at the outlet of the heated zone at the onset of significant void (OSV) conditions, caused by the increasing of the average velocity at that zone. That reduction of the pressure, at the outlet of the heated zone, decreases as the inlet (and also the outlet) temperature increases, due to the increasing of the length of the two phase flow at the unheated zone. That phenomenon is backed by photographing the flow in the unheated zone.     

Exergoeconomic analysis of condenser type heat exchangers

In this study, an exergoeconomic analysis of condenser type parallel flow heat exchangers is presented. Exergy losses of the heat exchanger and investment and operation expenses related to this are determined with functions of steam mass flow rate and water exit temperature at constant values of thermal power of the heat exchanger at 75240 W, cold water mass flow rate and temperature. The inlet temperature of water is 18 °C and exit temperatures of water are varied from 25 °C to 36 °C. The values of temperature and pressure of saturated steam in the condenser are given to be T_con=47 ° C and P_con=10.53 kPa. Constant environment conditions are assumed. Annual operation hour and unit price of electrical energy are taken into account for determination of the annual operation expenses. Investment expenses are obtained according to the variation of heat capacity rate and logarithmic mean temperature difference and also heat exchanger dimension determined for each situation. The present analysis is hoped to be useful in determining the effective parameters for the most appropriate exergy losses together with operating conditions and in finding the optimum working points for the condenser type heat exchangers.     

Experimental and theoretical investigation of an innovative evaporative condenser for residential refrigerator

In this study, an innovative, evaporative condenser for residential refrigerator was introduced. A vapor compression cycle incorporating the proposed evaporative condenser was tested to evaluate the cycle performance. To allow for evaporative cooling, sheets of cloth were wrapped around condenser to suck the water from a water basin by capillary effect. The thermal properties at the different points of the refrigeration cycle were measured for typical operating conditions. The experimental results showed that the condenser temperature increases 0.45 °C for each degree increase in evaporator temperature when the air velocity is 2.5 m/s, and the ambient condition is 29 °C and the relative humidity is 37.5%. Meanwhile, the condenser temperature increase is 0.88 °C in the case of air velocity 1.1 m/s and ambient conditions of 31 °C and relative humidity of 47.1%. A theoretical model for the evaporative condenser was developed, and validated by experimental results. The theoretical model showed that the evaporative condenser can operate at a condensing temperature of 20 C lower than that of the air-cooled condenser for heat flux of 150 W/m², and at air velocity 3 m/s. The effect of the different parameters on the condenser temperature was studied too.     

Influence of hydrodynamic instability on the heat transfer coefficient during condensation of R134a and R404A refrigerants in pipe mini-channels

This paper presents the results of an investigation of the influence of hydrodynamic instabilities on heat transfer intensity during the condensation of R134a and R404A refrigerants in pipe mini-channels. The heat transfer coefficient h is a measure of the effectiveness of the condensation process. It is particularly important to determine the value of the coefficient in the two-phase condensation area in a compact condenser. In other condenser areas (i.e., precooling of superheated vapor and subcooling of condensate), the heat efficiency is substantially smaller. Hydrodynamic instabilities of a periodic nature have an influence on size changes in these areas. A decrease in the heat transfer coefficient h in the two-phase area results in decreased intensity of the heat removal process in the whole condenser.The experimental investigations were based on the condensation of R134a and R404A refrigerants in horizontal pipe mini-channels with internal diameters of d = 0.64; 0.90; 1.40; 1.44; 1.92; 2.30 and 3.30 mm. Disturbances of the condensation process were induced with a periodic stop and a repetition of the flow of the refrigerant.In the range of frequencies, f = 0.25–5 Hz, of the periodically generated disturbances, an unfavorable influence on the intensity of the heat transfer during the condensation process in pipe mini-channels was identified. The reduction in the intensity of the heat transfer during the condensation process, which was induced with hydrodynamic instabilities, was presented in the form of the dependence of the heat transfer coefficient h on the vapor quality x and the frequencies f of the disturbances.The influence of the refrigerant, the diameter of the mini-channels and the frequency f on the damping phenomenon of the periodical disturbances in the pipe mini-channels was identified.     

Experimental study of a solid adsorption cooling system using flat-tube heat exchangers as adsorption bed

In this paper, a solid adsorption cooling system with silica gel as the adsorbent and water as the adsorbate was experimentally studied. To reduce the manufacturing costs and simplify the construction of the adsorption chiller, a vacuum tank was designed to contain the adsorption bed and evaporator/condenser. Flat-tube type heat exchangers were used for adsorption beds in order to increase the heat transfer area and improve the heat transfer ability between the adsorbent and heat exchanger fins. Under the standard test conditions of 80 °C hot water, 30 °C cooling water, and 14 °C chilled water inlet temperatures, a cooling power of 4.3 kW and a coefficient of performance (COP) for cooling of 0.45 can be achieved. It has provided a specific cooling power (SCP) of about 176 W/(kg adsorbent). With lower hot water flow rates, a higher COP of 0.53 can be achieved.     

Condensation from superheated vapor flow of R744 and R410A at subcritical pressures in a horizontal smooth tube

This paper presents experimentally determined heat transfer coefficients for condensation from a superheated vapor of CO₂ and R410A. The superheated vapor was flowed through a smooth horizontal tube with 6.1 mm ID under almost uniform temperature cooling at reduced pressures from 0.55 to 0.95, heat fluxes from 3 to 20 kW m^?2, and superheats from 0 to 40 K. When the tube wall temperature reaches the saturation point, the measured results show that the heat transfer coefficient gradually starts deviating from the values predicted by a correlation valid for single-phase gas cooling. This point identifies the start of condensation from the superheated vapor. The condensation starts earlier at higher heat fluxes because the tube wall temperature reaches the saturation point earlier. The heat transfer coefficient reaches a value predicted by correlations for condensation at a thermodynamic vapor quality of 1. The measured heat transfer coefficient of CO₂ is roughly 20–70% higher than that of R410A at the same reduced pressures. This is mainly because the larger latent heat and liquid thermal conductivity of CO₂, compared to that of R410A, increase the heat transfer coefficient.     

An experimental investigation on the thermal efficiency of fractal tree-like microchannel nets

In this paper, a fractal tree-like microchannel net heat sink (20 mm × 20 mm × 1.4 mm) for cooling of electronic chips was fabricated on a silicon wafers by advanced MEMS technology. The length, width and height of the entrance microchannel were 10 mm, 800 μm and 25 μm, respectively. The fractal dimension D and the circulation number m of the fractal tree-like microchannel net were 2 and 4, respectively. It is confirmed experimentally that the thermal efficiency (defined as heat transfer rate per unit power required) of such a fractal tree-like microchannel heat sink is much higher than that of the traditional parallel microchannel heat sink for the same heat transfer rate, the same temperature difference and the same inlet velocity.     

Numerical study of laminar heat transfer and pressure drop characteristics in a water-cooled minichannel heat sink

With the rapid development of the information technology (IT) industry, the heat flux in integrated circuit (IC) chips cooled by air has almost reached its limit about 100 W/cm². Some applications in high technologies require heat fluxes well beyond such a limitation. Therefore the search of a more efficient cooling technology becomes one of the bottleneck problems of the further development of IT industry. The microchannel flow geometry offers large surface area of heat transfer and a high convective heat transfer coefficient. However, it has been hard to implement because of its very high pressure head required to pump the coolant fluid though the channels. A normal channel could not give high heat flux although the pressure drop is very small. A minichannel can be used in heat sink with a quite high heat flux and a mild pressure loss. A minichannel heat sink with bottom size of 20 mm × 20 mm is analyzed numerically for the single-phase laminar flow of water as coolant through small hydraulic diameters and a constant heat flux boundary condition is assumed. The effects of channel dimensions, channel wall thickness, bottom thickness and inlet velocity on the pressure drop, thermal resistance and the maximum allowable heat flux are presented. The results indicate that a narrow and deep channel with thin bottom thickness and relatively thin channel wall thickness results in improved heat transfer performance with a relatively high but acceptable pressure drop. A nearly-optimized configuration of heat sink is found which can cool a chip with heat flux of 256 W/cm² at the pumping power of 0.205 W. The nearly-optimized configuration is verified by an orthogonal design. The simulated thermal resistance agrees quite well with the result of conventional correlations method with the maximum difference of 12%.     

Single-phase and two-phase cooling with an array of rectangular jets

Experiments were performed to explore the effects of jet width, impingement velocity, and inlet subcooling on the cooling performance of an array of three confined rectangular FC-72 and ethanol jets impacting a 3.0 cm × 3.0 cm heated surface. The single-phase heat transfer coefficient increased with increasing jet velocity and/or jet width. A correlation for single-phase cooling was constructed by dividing the flow into impingement zones and confinement channel flow regions that are dominated by wall jet flow. Increases in jet velocity, jet width, and/or subcooling broadened the single-phase region preceding the commencement of boiling and enhanced critical heat flux (CHF). A new correlation was developed which fits the CHF data with good accuracy. Overall, better cooling performance was realized for a given flow rate by decreasing jet width. Pressure drop was for the most part quite modest, even for the smallest jet width and highest velocity tested. Overall, these results prove the present cooling scheme is highly effective at maintaining fairly isothermal surface conditions, with spatial variations of less than 1.2 and 2.6 °C for the single-phase and boiling regions, respectively. These results demonstrate the effectiveness of the present jet-impingement scheme for thermal management of next generation electronics devices and systems.     

Experimental investigation of convective heat transfer coefficient during downward laminar flow condensation of R134a in a vertical smooth tube

This paper presents an experimental investigation of laminar film condensation of R134a in a vertical smooth tube having an inner diameter of 7–8.1 mm and a length of 500 mm. Condensation experiments were performed at mass fluxes of 29 and 263 kg m^?2 s^?1. The pressures were between 0.77 and 0.1 MPa. The heat transfer coefficient, film thickness and condensation rate during downward condensing film were determined. The results show that an interfacial shear effect is significant for the laminar condensation heat transfer of R134a under the given conditions. A new correlation for the condensation heat transfer coefficient is proposed for practical applications.     

Condensation heat transfer and pressure drop of R134a in annular helicoidal pipe at different orientations

Experimental investigations were conducted to determine the condensation heat transfer and pressure drop of refrigerant R134a in annular helicoidal pipe at three inclination angles. The experiments were performed with the Reynolds number of R134a ranging from 60 to 200, and that of cooling water from 3600 to 22 000; temperatures of R134a at 30 °C and 35 °C, and cooling water at 16 °C, 20 °C and 24 °C. The experimental results indicated that the refrigerant Nusselt number was larger at lower refrigerant saturation temperature, and would increase with the increase of mass flow rates of refrigerant and cooling water. It was found that the refrigerant heat transfer coefficient of annular helicoidal pipe could be two times larger than that of equivalent plain straight pipe when the refrigerant Reynolds number was larger than 140. Comparison with identical helicoidal pipe with opposite flow channel arrangement revealed that the refrigerant heat transfer rate was larger when the refrigerant was flowing in the annular section at the cooling water Reynolds number larger than 4000, but the pressure drop was always larger in this flow channel arrangement.     

Theoretical model for annular flow condensation in rectangular micro-channels

This study examines the pressure drop and heat transfer characteristics of annular condensation in rectangular micro-channels with three-sided cooling walls. A theoretical control-volume-based model is proposed based on the assumptions of smooth interface between the annular liquid film and vapor core, and uniform film thickness around the channel’s circumference. Mass and momentum conservation are applied to control volumes encompassing the liquid film and the vapor core separately. The model accounts for interfacial suppression of turbulent eddies due to surface tension with the aid of a new eddy diffusivity model specifically tailored to shear-driven turbulent films. The model predictions are compared with experimental pressure drop and heat transfer data for annular condensation of FC-72 along 1 × 1 mm² parallel channels. The condensation is achieved by rejecting heat to a counterflow of water. The data span FC-72 mass velocities of 248–367 kg/m² s, saturation temperatures of 57.8–62.3 °C, qualities of 0.23–1.0, and water mass flow rates of 3–6 g/s. The data are also compared to predictions of previous separated flow mini/micro-channel and macro-channel correlations. While some of the previous correlations do provide good predictions of the average heat transfer coefficient, they fail to capture axial variation of the local heat transfer coefficient along the channel. The new model accurately captures the pressure drop and heat transfer coefficient data in both magnitude and trend, evidenced by mean absolute error values of 3.6% and 9.3%, respectively.     

Measurements of jet flows impinging into a channel containing a rod bundle using dynamic PIV

In this investigation, full-field velocity profiles of jet flows impinging into a rod bundle were measured using Particle Image Velocimetry (PIV) technique in a facility with Matched-Index of Refraction. The test section consisted of a 1 m long rectangular channel with a cross-sectional area of 76.2 × 76.2 mm². Water was pumped through either single or double jets located on the top wall of the channel. The Reynolds numbers of the inlet jets were varied from 4470 to 13,400. Inside the channel adjacent to the inlet jets, a rod bundle was configured with 29 staggered tubes of 10.67 mm outside diameter (O.D.). The time-resolved velocity fields were obtained for regions within the rod bundle from which the vorticity fields and the turbulent characteristics of those regions were calculated. The results are useful for developing new models to help predict the flow patterns in the lower plenum of Gas Cooled Reactors.