HC-290 (Propane) Vaporisation Inside a Brazed Plate Heat Exchanger

This paper presents the heat transfer coefficients measured during HC-290 (Propane) vaporisation inside a brazed plate heat exchanger: the effects of heat flux, saturation temperature (pressure) and outlet conditions are investigated. The heat transfer coefficients show weak sensitivity to saturation temperature (pressure) and great sensitivity to heat flux and outlet conditions. The saturated boiling experimental heat transfer coefficients are compared with two well-known equations for nucleate boiling (Cooper (1984) and Gorenflo (1993)). The mean absolute percentage deviation between experimental and calculated heat transfer coefficients is 26.9% and 16.6% for Cooper (1984) and Gorenflo (1993) equation respectively. The heat transfer measurement has been complemented with IR thermography in order to quantify the portion of the heat transfer surface affected by vapour super-heating

Refrigerant R410A Vaporisation Inside a Small Brazed Plate Heat Exchanger

This paper presents the experimental heat transfer coefficients and pressure drop measured during refrigerant R410A vaporisation inside a small brazed plate heat exchanger: the effects of heat flux, refrigerant mass flux, saturation temperature and outlet conditions are investigated. The experimental results are reported in terms of refrigerant side heat transfer coefficients and frictional pressure drop. The heat transfer coefficients show high sensitivity both to heat flux and outlet conditions and weak sensitivity to saturation temperature. The frictional pressure drop shows high sensitivity to refrigerant mass flux and weak sensitivity both to saturation temperature and outlet conditions. The experimental heat transfer coefficients are also compared with two well-known correlations for nucleate pool boiling: a fair agreement is found

Experimental Analysis of R134a and R1234ze(E) Flow Boiling Inside a Roll Bond Evaporator

Roll bond type evaporator is one of the most widely used technology in household refrigerators. Despite that, only few works that analyze the performance of this component are available in literature. Furthermore, no evidence is given to the impact on the heat transfer performance when substituting the original HFC refrigerant with an HFO inside the same evaporator. This paper presents an experimental study of R134a and R1234ze(E) inside an off the shelf roll bond evaporator, commonly used for small size domestic refrigerators. The evaporator was mounted inside a climate dark chamber where ambient temperature and humidity were maintained stable during the tests. To control the inlet conditions (evaporation temperature, inlet quality, refrigerant mass flow rate) it was used a water cooled miniature scale vapor cycle system with R134a and R1234ze(E) as working fluids. By means of an IR-thermo-camera, the whole roll bond temperature field was investigated under different working conditions. 15 thermocouples were collocated on the back of the evaporator to verify the temperatures reported in the images collected by the IR thermo-camera. From these pictures it was possible to delineate the super heating region and to point out the areas of the evaporator in which heat transfer is less efficient depending on the fluid and on the working conditions. During the experimental tests the refrigerant mass flow rate was varied by regulating the compressor speed, while ambient temperature and evaporation temperature were kept as constant. The data acquired from the vapor cycle system (i.e. condensation and evaporation pressure, evaporator inlet quality, vapour superheating, refrigerant mass flow rate) coupled with the IR thermo-camera images allowed to evaluate the behavior and the efficiency of the roll bond. Since the data were collected maintaining the same operating conditions in term of ambient temperature and humidity, heat flow rate and evaporation temperature, it was possible to highlight differences among the two refrigerants in term of mass flux. Furthermore, on the basis of the IR images and of the thermocouples measurements, an average heat transfer coefficient was defined and determined both on the air and the refrigerant side. The average heat transfer coefficients of the two refrigerants are compared and outlined in the paper

HFO1234ze(E) Boiling Inside a Brazed Plate Heat Exchanger

HFC134a has been probably the most important refrigerant of the two past decades as it dominated the application in domestic refrigeration, mobile air conditioning and large chillers and it took part as component in several refrigerant mixtures such as HFC404A, and HFC407C. Unfortunately HFC134a exhibits a relatively large Global Warming Potential (GWP), and it will be subjected to a gradual reduce in the use up to a complete phase out in the next future according to the different national and international regulations. The HydroFluoroOlefins (HFO) refrigerants HFO1234yf and HFO1234ze(E) seem to be the most promising substitutes for HFC134a as they exhibit very low GWP values (1 or less) together with pressure and volumetric properties closely near to those of HFC134a. The unique drawback of HFO refrigerants seems to be their mild flammability. The Brazed Plate Heat Exchangers (BPHE), which involve a reduction of the refrigerant charge of one order of magnitude as compared to the traditional tubular heat exchangers, are particularly interesting for limiting the risk of flammable or mildly flammable refrigerants such as HFO1234ze(E). In fact the first attempt to reduce the risk of flammable refrigerants is to decrease the refrigerant charge. This paper presents the experimental heat transfer coefficients and pressure drop measured during HFO1234ze(E) boiling inside a small BPHE: the effects of heat flux, refrigerant mass flux, saturation temperature (pressure) and outlet conditions are investigated. The evaporator tested is a BPHE consisting of 10 plates, 72 mm in width and 310 mm in length, which present a macro-scale herringbone corrugation with an inclination angle of 65° and a corrugation amplitude of 2 mm. The experimental tests have been carried out at three different saturation temperatures (10, 15 and 20°C) and four different evaporator outlet conditions (vapour quality around 0.80 and 1.00, vapour super-heating around 5 and 10°C), whereas the inlet vapour quality ranges between 0.2 and 0.3. The refrigerant mass flux ranges from 11 to 31 kg/m2s and the heat flux from 4 to 17 kW/m2. The experimental results are reported in terms of refrigerant side heat transfer coefficients and frictional pressure drop. The heat transfer coefficients show great sensitivity to heat flux, outlet conditions and fluid properties and weak sensitivity to saturation temperature (pressure). The frictional pressure drop shows a linear dependence on the kinetic energy per unit volume of the refrigerant flow and therefore a quadratic dependence on refrigerant mass flux. The heat transfer and pressure drop measurements are complemented with an IR thermography analysis carried out during the vaporisation tests. The saturated boiling heat transfer coefficients were compared with a new model for refrigerant boiling inside BPHE (Longo et al., 2015): the mean absolute percentage deviation between calculated and experimental data is 7.2%. The present data points were also compared with those of HFC134a and HFO1234yf previously measured inside the same BPHE under the same operating conditions: HFO1234ze(E) exhibits heat transfer coefficients very similar to HFC134a and HFO1234yf and frictional pressure drops slightly higher than HFC134a and HFO1234yf

R1233zd(E) and R245fa Flow Boiling Heat Transfer and Pressure Drop inside a 4.2 mm ID Microfin Tube

This paper presents R1233d(E) and R245fa flow boiling heat transfer and pressure drop measurements inside a mini microfin tube having internal diameter at the fin tip equal to 4.2 mm, 40 fins 0.15 mm high, and a helix angle of 18°. The tube was brazed inside a copper plate and electrically heated from the bottom. Sixteen T-type thermocouples were located in the copper plate to monitor the wall temperature. The experimental measurements were carried out at a constant mean saturation temperature of 30 °C, by varying the refrigerant mass velocity between 100 kg m-2 s-1 and 300 kg m-2 s-1, the vapour quality, and the heat flux from 15 to 90 kW m-2. The experimental results are here presented in terms of two-phase heat transfer coefficient, onset dryout vapour quality, and frictional pressure drop. Moreover, R1233zd(E) has been proposed as low GWP alternative to R245fa. In this paper, the two fluids performance is compare under the same working conditions and commented. Finally, the experimental measurements were used to assess a few selected models for boiling heat transfer coefficient and frictional pressure drop estimations available in the open literature for microfin tubes

R134a Flow Boiling Heat transfer on an Electrically Heated Carbon/Carbon Surface

With the increase of heat flux densities following the Moore’s law, electronic cooling challenge is focused on the high heat flux to be dissipated by the operating fluid and more and more efficient heat spreaders, dissipators, and compact heat exchangers are in great demand for various applications. Considering the device efficiency, the boiling heat transfer ensures very high heat transfer coefficients, which can even be improved via specific surface treatments that have been shown to be very effective. In particular, several authors, experimentally demonstrated the interesting enhancement capabilities of microparticles coatings on the Critical Heat Flux. Furthermore, the recent work on nanoscale domain has led to new concepts for surface modification. In the last decade, nano-structured materials (i.e. nanowires coatings, nanoporous layers, Carbon Nano Tube arrays, etc.) have been proved to enhance the boiling heat transfer. Unfortunately, almost all of this kind of surface treatments fail when scaled up to industrial implementation because of the relatively high costs and complex operations involved. Furthermore, compactness and lightness of cooling systems are becoming even more challenging design constraints leading the research efforts towards new light and efficient materials. In this scenario, the Carbon/Carbon material appears to be a viable option for future thermal management devices because it exploits interesting properties having a low density and a high thermal conductivity; moreover, it is already used in many industrial applications where it is shaped in various forms even complex. This paper presents the experimental measurements carried out during flow boiling heat transfer of R134a on a Carbon/Carbon surface. The test section with the Carbon/Carbon sample, is electrically heated from the bottom and it is instrumented with 18 wall thermocouples to monitor the temperature distribution at an imposed heat fluxes of 50 kW m-2, and refrigerant mass flow rates from 50 to 200 kg m-2 s-1, at constant saturation temperature of 30 °C. The sample is tested in a new experimental facility built at the Nano Heat Transfer Lab of the Department of Management and Engineering of the University of Padova especially designed to study the flow boiling heat transfer process on innovative materials and enhanced micro- and nano-structured surfaces.

R134a And Its Low GWP Substitutes R1234yf And R1234ze(E) Flow Boiling Inside A 4mm Horizontal Smooth Tube

The substitution of HFC134a with low GWP refrigerants is one of the most important challenge for refrigeration and air conditioning. The possible substitutes include natural refrigerants, such as HC600 (Butane) and HC600a (Isobutane), and also synthetic refrigerants, such as HFO1234yf and HFO1234ze(E). The HC refrigerants exhibit very low GWP, 3 and 4 HC600a and HC600 respectively, good thermodynamic and transport properties, and pressure and volumetric performance very similar to HFC134a. The major drawback of HC refrigerants is their high flammability, being classified in class A3 according to ASHRAE classification. Also the HFO refrigerants present a mild flammability, being classified in class A2L. In fact, it is very difficult to found low GWP substitutes for traditional HFC refrigerants with no flammability, as a weak chemical stability and / or a big chemical reactivity are presuppositions for low GWP. Both HFO1234yf and HFO1234ze(E) seem to be very promising as substitute for HFC134a, showing a GWP lower than 1 together with pressure and volumetric properties closely near to those of HFC134a. This paper presents the comparative analysis of HFC134a HFO1234yf and HFO1234ze(E) during saturated flow boiling inside a 4 mm horizontal smooth tube: the effects of heat flux, refrigerant mass flux, mean vapour quality and saturation temperature (pressure) are investigated separately to rank the superposed effects of different heat transfer regimes (nucleate boiling or/and forced convection boiling). The experimental tests were carried out at three different saturation temperatures (10, 15, and 20 °C) at increasing vapour quality up to incipient dryout to evaluate the specific contribution of heat flux, refrigerant mass flux, mean vapour quality, and saturation temperature (pressure). The refrigerant mass flux ranges from 200 to 600 kg m-2s-1 and the heat flux from 15 to 30 kW m-2. The experimental measurements were reported in term of boiling heat transfer coefficients and frictional pressure drops. Heat transfer coefficients have a positive slope versus vapour quality and the slope increases with refrigerant mass flux and decreases with heat flux. Saturation temperature (pressure), refrigerant mass flux and mean vapour quality have a remarkable impact on the frictional pressure drop, whereas the effect of heat flux appears marginal or negligible. Convective boiling seems to be the prevailing heat transfer regime in present experimental tests. HFO1234ze(E) and HFO1234yf exhibit heat transfer coefficients and pressure drops similar to HFC134a. Present heat transfer coefficients and pressure drops were also compared against different correlations for refrigerant boiling inside tube. The universal correlation proposed by Kim and Mudawar (2014) and the Friedel (1979) correlation show the best performance in predicting heat transfer coefficients and pressure drops, respectively

HFO1234ze(E) And HFC134a Flow Boiling Inside a 4mm Horizontal Smooth Tube

Nowadays, the substitution of HFC134a with low GWP refrigerants is one of the most important challenge for refrigeration and air conditioning. The possible substitutes include natural refrigerants, such as HC600 (Butane) and HC600a (Isobutane), and also synthetic refrigerants, such as HFO1234yf and HFO1234ze(E). The HC refrigerants exhibit very low GWP, 3 and 4 HC600a and HC600 respectively, good thermodynamic and transport properties, and pressure and volumetric performance very similar to HFC134a. The major drawback of HC refrigerants is their high flammability, being classified in class A3 according to ASHRAE classification. Also the HFO refrigerants present a mild flammability, being classified in class A2L. In fact it is very difficult to found low GWP substitutes for traditional HFC refrigerants with no flammability, as a weak chemical stability and / or a big chemical reactivity are presuppositions for low GWP. In particular HFO1234ze(E) seems to be very promising as substitute for HFC134a, showing a GWP lower than 1 together with pressure and volumetric properties closely near to those of HFC134a. This paper presents the comparative analysis of HFC134a and HFO1234ze(E) during saturated flow boiling inside a 4 mm horizontal smooth tube: the effects of heat flux, refrigerant mass flux, mean vapour quality and saturation temperature (pressure) are investigated separately to rank the superposed effects of different heat transfer regimes (nucleate boiling or/and forced convection boiling). The experimental tests were carried out at three different saturation temperatures (10, 15, and 20 °C) at increasing vapour quality up to incipient dryout to evaluate the specific contribution of heat flux, refrigerant mass flux, mean vapour quality, and saturation temperature (pressure). The refrigerant mass flux ranges from 200 to 600 kg m-2s-1 and the heat flux from 15 to 30 kW m-2. The experimental measurements were reported in term of boiling heat transfer coefficients and frictional pressure drops. The heat transfer coefficients have a positive slope versus vapour quality and the slope increases with refrigerant mass flux and decreases with heat flux. Saturation temperature (pressure), refrigerant mass flux and mean vapour quality have a remarkable impact on the frictional pressure drop of both HFO1234ze(E) and HFC134a whereas the effect of heat flux appear marginal or negligible. Convective boiling seems to be the prevailing heat transfer regime in present experimental tests. HFO1234ze(E) exhibits heat transfer coefficients similar to HFC134a and slightly higher frictional pressure drops. Present heat transfer coefficients were compared against different heat transfer correlations for refrigerant boiling inside tube. The universal correlation proposed by Kim and Mudawar (2014) shows the best performance with a mean absolute percentage deviation of 6.1% both for HFO1234ze(E) and HFC134a data, respectively. Present frictional pressure drops were compared against different correlation for two-phase pressure drop inside tube: Friedel (1979) correlation shows the best performance with a mean absolute percentage deviation of 11.7% and 12.6% for HFO1234ze(E) and HFC134a respectively

R134a and its low GWP substitutes R1234yf and R1234ze(E) condensation inside a 4mm horizontal smooth tube

The substitution of HFC134a with low GWP refrigerants is one of the most important challenge for refrigeration and air conditioning. The possible substitutes include natural refrigerants, such as HC600 (Butane) and HC600a (Isobutane), and also synthetic refrigerants, such as HFO1234yf and HFO1234ze(E). The HC refrigerants exhibit very low GWP, 3 and 4 HC600a and HC600 respectively, good thermodynamic and transport properties, and pressure and volumetric performance very similar to HFC134a. The major drawback of HC refrigerants is their high flammability, being classified in class A3 according to ASHRAE classification. Also the HFO refrigerants present a mild flammability, being classified in class A2L. In fact it is very difficult to found low GWP substitutes for traditional HFC refrigerants with no flammability, as a weak chemical stability and / or a big chemical reactivity are presuppositions for low GWP. Both HFO1234yf and HFO1234ze(E) seem to be very promising as substitute for HFC134a, showing a GWP lower than 1 together with pressure and volumetric properties closely near to those of HFC134a. This paper presents the comparative analysis of HFC134a HFO1234yf and HFO1234ze(E)during saturated condensation inside a 4 mm horizontal smooth tube. The experimental tests were carried out at three different saturation temperatures (30, 35, and 40 °C) at decreasing vapour quality up to sub-cooled liquid condition, to evaluate the specific contribution of refrigerant mass flux, mean vapour quality, and saturation temperature. The refrigerant mass flux ranges from 100 to 600 kg m-2 s-1. The experimental measurements were reported in term of condensation heat transfer coefficients and frictional pressure drops and plotted in non-dimensional co-ordinates showing the heat transfer factor and the friction factor against the equivalent Reynolds number. A transition point from gravity dominated and forced convection condensation was found for an equivalent Reynolds number around 10000-20000. HFO1234yf and HFO1234ze(E) exhibit heat transfer coefficients and pressure drops similar to HFC134a and they seem to be valuable long term low GWP substitutes for HFC134a. The experimental heat transfer coefficients in the forced convection condensation regime were very well predicted by the Akers et al. (1959) model, whereas the Friedel (1979) correlation was able to reproduce the frictional pressure drop data both in gravity dominated and forced convection condensation regimes

HFO1234ze(Z) saturated vapour condensation inside a brazed plate heat exchanger

All commonly used Hydro-Fluoro-Carbon (HFC) refrigerants have a high Global Warming Potential (GWP), higher than 1000, and some countries have already enacted legislative measures towards a limitation in the use or a gradual phase-out of HFCs. HFO1234ze(Z) was identified as a new low GWP refrigerant, which has the potential to be a global sustainable solution particularly for heat pump application. HFO1234ze(Z) is a pure compound which exhibits low pressure and is classified by ANSI / ASHRAE Standard 34 (2010) as class A2L refrigerants (lower flammability and lower toxicity). Therefore, it can be used in direct expansion systems without the need for a secondary loop as alternative for HFC236fa and HC600a. This paper presents the experimental heat transfer coefficients and pressure drop measured during HFO1234ze(Z) saturated vapour condensation inside a small commercial BPHE: the effects of refrigerant mass flux and saturation temperature were investigated. The experimental facility consists of a refrigerant loop, a water-glycol loop and two water loops. In the first loop the refrigerant is pumped from the sub-cooler into the evaporator where it is evaporated to achieve the set condition at the condenser inlet. The refrigerant goes through the condenser where it is condensed and then it comes back to the post-condenser and the sub-cooler. A variable speed volumetric pump varies the refrigerant flow rate and a bladder accumulator, connected to a nitrogen bottle and a pressure regulator, controls the operating pressure in the refrigerant loop. The second loop is able to supply a water-glycol flow at a constant temperature used to feed the sub-cooler and the post-condenser. A set of 42 saturated vapour condensation data points with refrigerant down-flow and water up-flow was carried out at four different saturation temperatures: 25, 30, 35 and 40°C. The experimental results were presented in terms of heat transfer coefficients and frictional pressure drop. The heat transfer coefficients show weak sensitivity to saturation temperature and great sensitivity to refrigerant mass flux. At low refrigerant mass flux ( \u3c 20 kg/m2s) the heat transfer coefficients are independent of mass flux and condensation is controlled by gravity. For higher refrigerant mass flux ( \u3e 20 kg/m2s) the heat transfer coefficients depend on mass flux and forced convection condensation occurs. The frictional pressure drop shows a linear dependence on the kinetic energy per unit volume of the refrigerant flow and therefore a quadratic dependence on the refrigerant mass flux. HFO1234ze(Z) shows heat transfer coefficients higher both than HC600a and HFC236fa and frictional pressure drop higher than HFC236fa and lower than HC600a. The experimental results were compared against theoretical models for condensation heat transfer coefficients (Nusselt 1916 and Akers et al. 1959) and a new linear correlation for two-phase frictional pressure drop was presented