Mass Flux Measurement of Two Phase Dense Spray Using a Coupled Impulse Probe and PDPA Technique

Mass flux and void fraction measurement in a multiphase dense spray is a challenging task. The Phase Doppler Particle Anemometer (PDPA) cannot provide accurate mass flux measurements in a highly turbulent multiphase spray due to the presence of non-spherical and multiple droplets in the probe volume. A combined measurement of momentum data from the impulse probe and velocity data from the PDPA provides a fairly reasonable estimate of mass flux data in the two phase spray envelope. Experimental results show that mass flux at 60Dn (Dn= nozzle diameter of 3.10 mm) downstream of a horizontal nozzle tip is 0.033 kg/s, 0.034 kg/s and 0.0005 kg/s obtained from the theoretical value, impulse probe method and PDPA technique, respectively. This study will help answer some of the fundamental questions about the mass flux distribution in the two phase dense spray, which will aid in the improvement of the multiphase atomization design process in industrial applications

A review of refrigerant R1234ze(E) recent investigations

[EN] Climate change is demonstrated through global surface temperatures increase in the last century. To stop this phenomenon, new regulations that ban or tax greenhouse gas fluids (HFC among them) have been approved. In the medium term, only low-GWP refrigerants will be permitted in developed countries. HFO fluids and most used HFCs as refrigerants in HVACR systems possess similar thermophysical properties. Among them, one of the most promising is R1234ze(E). This refrigerant presents good environmental properties and can be used in most of HVACR applications, pure or mixed with HFC or natural refrigerants (mainly CO2). This paper collects the most relevant research about R1234ze(E) thermophysical and compatibility properties, heat transfer and pressure drop characteristics, and vapor compression system performance; separating those works that consider R1234ze(E) pure or blended. Once the available literature is analyzed, it can be concluded that pure R1234ze(E) is a good option only in new HVACR systems. Nevertheless, if it is combined with other refrigerants, the final GWP value is also considerably reduced, maintaining efficiency parameters at levels that allow them to replace R134a, R404A or R410A in existing systems with minor modifications.The authors thankfully acknowledge the “Ministerio de Educación, Cultura y Deporte” (Grant Number FPU12/02841) for supporting this work through “Becas y Contratos de Formación de Profesorado Universitario del Programa Nacional de Formación de Recursos Humanos de Investigación del ejercicio 2012”.Mota-Babiloni, A.; Navarro-Esbrí, J.; Molés, F.; Barragán Cervera, Á.; Peris, B.; Verdú Martín, GJ. (2016). A review of refrigerant R1234ze(E) recent investigations. Applied Thermal Engineering. 95:211-222. https://doi.org/10.1016/j.applthermaleng.2015.09.055S2112229

Experimental study on flow boiling heat transfer of multiport tubes with R245fa and R1234ze(E)

The upward flow boiling experiments were carried out in a flat aluminum extruded multiport tube, which is composed of 7 parallel rectangular channels (1.1 mm × 2.1 mm) with hydraulic diameter of 1.4 mm. Two refrigerants, R245fa and R1234ze, the latter a recent environmentally safe refrigerant proposed as a potential replacement for R134a, were tested. A new hot water heating technique that accounts for either uniform or non-uniform local heat flux distribution along the channel was developed to obtain and reduce the data. Effects of heat flux, mass flux, vapor quality, and saturation temperature on flow boiling heat transfer in multiport tubes were considered. Finally, the experimental results were compared with some well-known correlations to evaluate the capabilities of existing prediction methods. The analysis shows that the three-zone model for slug flows works well for that subset of test results, utilizing the apparent surface roughness in place of the dryout thickness in the model, as has been previously done for silicon, stainless steel and copper microchannels with measured surface roughnesses

Flow boiling and frictional pressure gradients in plate heat exchangers. Part 2: Comparison of literature methods to database and new prediction methods

In the second part of this study a sensitivity analysis on the prediction methods is performed to consider the effect of plate geometry on thermal hydraulic performance and an extensive comparison of all the two-phase pressure drop and flow boiling heat transfer prediction methods available in the open literature are also provided versus the large diversified database presented in Part 1. The experimental databank, from numerous independent research studies, is then utilized to develop the new prediction methods to evaluate local heat transfer coefficients and pressure drops. These new methods were developed from 1903 heat transfer and 1513 frictional pressure drop data points (3416 total), respectively, and were proved to work better over a very wide range of operating conditions, plate designs and fluids (including ammonia). The prediction for flow boiling heat transfer coefficients was broken down into separate macro- and micro-scale methods. (C) 2015 Elsevier Ltd and International Institute of Refrigeration. All rights reserved

Flow boiling and frictional pressure gradients in plate heat exchangers. Part 1: Review and experimental database

This two-part paper presents an overview of evaporation heat transfer mechanisms, a review of the experimental and prediction methods and a creation of a consolidated multi-lab database of 3601 data points and provides a detailed comparison of all the prediction methods to this broad database and finally proposes new prediction methods for the local heat transfer coefficient and the frictional pressure gradient of flow boiling within plate heat exchangers. Specifically, in Part 1, a description of the complex geometry of plate heat exchangers and an introduction to their major applications are described, followed by an extensive literature survey of experimental studies and associated prediction methods. While many prediction methods are found to work in the literature, the results of this study show that these methods have only been compared to their original data, but have not been vetted against a large database covering many fluids, plate designs and test conditions. (C) 2015 Elsevier Ltd and International Institute of Refrigeration. All rights reserved

MHD flows of UCM fluids above porous stretching sheets using two-auxiliary-parameter homotopy analysis method

The performance of a two-auxiliary-parameter homotopy analysis method (HAM) is investigated in solving laminar MHD flow of an upper-convected Maxwell fluid (UCM) above a porous isothermal stretching sheet. The analysis is carried out up to the 20th-order of approximation, and the effect of parameters such as elasticity number, suction/injection velocity, and magnetic number are studied on the velocity field above the sheet. The results will be contrasted with those reported recently by Hayat et al. [Hayat T, Abbas Z, Sajid M. Series solution for the upper-convected Maxwell fluid over a porous stretching plate. Phys Lett A 358;2006:396–403] obtained using a third-order one-auxiliary-parameter homotopy analysis method. It is concluded that the flow reversal phenomenon as predicted by Hayat et al. (2006) may have arisen because of the inadequacies of using just one-auxiliary-parameter in their analysis. That is, no flow reversal is predicted to occur if instead of using one-auxiliary-parameter use is made of two auxiliary parameters together with a more convenient set of base functions to assure the convergence of the series used to solve the highly nonlinear ODE governing the flow

Experimental study on condensation heat transfer in vertical minichannels for new refrigerant R1234ze(E) versus R134a and R236fa

Experimental condensation heat transfer data for the new refrigerant R1234ze(E), trans-1,3,3,3-tetrafluoropropene, are presented and compared with refrigerants R134a and R236fa for a vertically aligned, aluminum multi-port tube. Local condensation heat transfer measurements with such a multi-microchannel test section are very challenging due to the large uncertainties related to the heat flux estimation. Presently, a new experimental test facility was designed with a test section to directly measure the wall temperature along a vertically aligned aluminum multi-port tube with rectangular channels of 1.45 mm hydraulic diameter. Then, a new data reduction process was developed to compute the local condensation heat transfer coefficients accounting for the non-uniform distribution of the local heat flux along the channels. The condensation heat transfer coefficients showed the expected decrease as the vapor quality decreased (1.0-0.0) during the condensation process, as the mass velocity decreased (260-50 kg m(-2) s(-1)) and as the saturation temperature increased (25-70 degrees C). However, the heat transfer coefficients were not affected by the condensing heat flux (1-62 kW m(-2)) or by the entrance conditions within the tested range. It was found that the heat transfer performance of R1234ze(E) was about 15-25% lower than for R134a but relatively similar to R236fa. The experimental data were then compared with leading prediction methods from the literature for horizontal channels. In general, the agreement was poor, over-predicting the high Nusselt number data and under-predicting the low Nusselt number data, but capturing the mid-range quite well. A modified correlation was developed and yielded a good agreement with the current database for all three fluids over a wide range of operating conditions. (C) 2010 Elsevier Inc. All rights reserved