An Investigation of Void Fraction in the Stratified/Annular Flow Regions in Smooth, Horizontal Tubes

Refrigerants R134a and R410A have been used for void fraction measurements in smooth horizontal tubes with diameters between 4mm and 7mm. Quality and mass flux were varied from 5% to 90% and 75 kglm2-s to 700 kglm2-s, respectively. Two test loops, one for condensing flows at 35C and the other for evaporating flows at 5C, were used in the investigation. Results show that near the transition from annular to stratified flow void fraction changed from viscousinertial dependence to gravitational-inertial dominated dependence. An important feature observed is the annular flow region's relative insensitivity to mass flux while the border region between annular and stratified flows is characterized by strong mass flux dependence.Air Conditioning and Refrigeration Project 7

EXPERIMENTS DURING FLOW BOILING OF A R22 DROP-IN: R422D ADIABATIC PRESSURE GRADIENTS

R22, the HCFC most widely used in refrigeration and air-conditioning systems in the last years, is phasing-out. R422D, a zero ozone-depleting mixture of R125, R134a and R600a (65.1%/31.5%/3.4% by weight, respectively), has been recently proposed as a drop-in substitute. For energy consumption calculations and temperature control, it is of primary importance to estimate operating conditions after substitution. To determine pressure drop in the evaporator and piping line to the compressor, in this paper the experimental adiabatic pressure gradients during flow boiling of R422D are reported for a circular smooth horizontal tube (3.00 mm inner radius) in a range of operating conditions of interest for dry-expansion evaporators. The data are used to establish the best predictive method for calculations and its accuracy: the Moreno-Quibèn and Thome method provided the best predictions for the whole database and also for the segregated data in the annular flow regime. Finally, the experimental data have been compared with the adiabatic pressure gradients of both R22 and its much used alternative R407C available in the literature

Two-phase heat transfer in small passages and microfinned surfaces - Fundamentals and applications

Micro channels and internally finned tubes are increasingly being utilized in the evaporators and condensers of refrigeration systems. The adoption of such geometries in the development of micro-cooling systems is first discussed in this paper. Recent work on flow boiling heat transfer and condensation in small to micro passages as well as on microfinned surfaces is then presented. The complex effect of diameter size on flow boiling patterns and heat transfer and correlations currently available in literature are summarized. Condensation in microfinned tubes and microchannels is then discussed

Nano-refrigerants and nano-lubricants in refrigeration : synthesis, mechanisms, applications, and challenges

Addressing global energy security and environmental concerns, the utilization of nano-refrigerants and nano-lubricants has emerged as an innovative path for enhancing heat transfer. This research focuses on enhancing the thermophysical properties, heat transfer efficiency, and tribological characteristics of nanofluids—nanoparticles dispersed in refrigerants or lubricants. These nanofluids have demonstrated significant potential in applications such as cooling, air conditioning systems, and heat transfer equipment including pumps and pipes. A comprehensive understanding of parameters like thermal conductivity, viscosity, pressure drop, pumping power, and energy performance is delivered, with the aim of enhancing the overall efficiency of refrigeration systems, particularly the coefficient of performance (COP). Additionally, the review covers existing research on flow and pool boiling heat transfer, nano-lubricant tribological enhancement, and nano-refrigerant condensation. The study also addresses the challenges associated with the use of nano-refrigerants and nano-lubricants and offers a prospective outlook for their usage. These novel nanofluids are anticipated to emerge as effective solutions for increasing the COP and reducing energy consumption in the industrial sector, thus extending beyond the scope of previous efforts in this field. This review could serve as a valuable resource for a broad audience interested in this novel approach to energy efficiency

Evaluation of R448A and R450A as Low-GWP alternatives for R404A and R134a using a micro-fin tubes evaporator model

[EN] When retrofitting new refrigerants in an existing vapour compression system, their adaptation to the heat exchangers is a major concern. R450A and R448A are commercial non-flammable mixtures with low GWP developed to replace the HFCs R134a and R404A, fluids with high GWP values. In this work the evaporator performance is evaluated through a shell-and-microfin tube evaporator model using R450A, R448A, R134a and R404A. The accuracy of the model is first studied considering different recently developed micro-fin tube correlations for flow boiling phenomena. The model is validated using experimental data from tests carried out in a fully monitored vapour compression plant at different refrigeration operating conditions. The main predicted operational parameters such as evaporating pressure, UArp, and cooling capacity, when compared with experimental data, fit within 10% using the Akhavan-Behabadi et al. correlation for flow boiling. Results show that R450A and R404A are the refrigerants in which the model fits better, even though R448A and R134a predictions are also accurate. (C) 2015 Elsevier Ltd. All rights reserved.The authors thankfully acknowledge "Ministerio de Educacion, Cultura y Deporte" (Grant number FPU12/02841) for supporting this work through "Becas y Contratos de Formacion de Profesorado Universitario del Programa Nacional de Formacion de Recursos Humanos de Investigacion del ejercicio 2012". Finally the linguistic support of Irene I. Elias-Miranda is appreciated.Mendoza Miranda, JM.; Mota-Babiloni, A.; Navarro Esbri, J. (2016). Evaluation of R448A and R450A as Low-GWP alternatives for R404A and R134a using a micro-fin tubes evaporator model. Applied Thermal Engineering. 98:330-339. https://doi.org/10.1016/j.applthermaleng.2015.12.064S3303399

Experimental Investigation Of A New Low-Approach Evaporator With Reduced Refrigerant Charge

The impact of the refrigerant charge is growing more and more due to the high cost of the new synthetic refrigerants (HFO) and the increasing cost of the older refrigerants. In the field of the air conditioning, for the centrifugal systems using flooded evaporators it is useful to find alternative heat exchangers with lower refrigerant charge but the same performance. Namely, it is desired a low temperature approach between the water outlet and the evaporating refrigerant, both for increasing the COP of the machine and for reaching low pressure ratios, typically required in the centrifugal compressors. Nowadays, falling film or spray evaporators start to be considered and used. In general they require a complicated distribution system for the liquid inlet and for the flash gas dissipation, and a big volume of the shell in order to avoid liquid flowing to the compressor. Moreover, the performance at partial loads is problematic, as in general it is difficult to maintain wet all the tubes when varying the operating conditions. The present work describes a shell-and-tube evaporator with a completely new design, named “Hybrid Film Evaporator”, where the refrigerant evaporates on the external of the tubes, and the water is cooled flowing inside the tubes. In it there is a combination of a falling down motion of the liquid to be evaporated and a flooding of all the rows of tubes. The liquid is automatically redistributed on each row, and all the surface is always involved in the heat transfer. Two prototypes are investigated, one with four passes and one with six passes on the water side, in a oil-free centrifugal chiller. Both are 1 m long, and their shell diameter is 610 mm. The experimental analysis consists on the measurement of the capacity (ranging from 250 to 450 kW) in conjunction with the evaporating temperature (ranging from 5.5 to 6.5 °C) with R134a as refrigerant fluid. The water outlet temperature is kept at a constant value of 7 °C, and two different sets of experimental data are obtained, keeping constant either the water inlet temperature (at 12 °C) or the water flowrate (at 50 or 65 m³/h). The heat transfer coefficients (both global and external) are then studied. In addition, a flooded evaporator with the same overall geometry is experimentally studied and all the performance parameters are compared to the above ones. The conclusion is that the hybrid film evaporator reaches the same average heat transfer coefficient, and then the same temperature approach at a given heat flux, of a flooded evaporator with the same surface area, saving the refrigerant charge of a quantity between 30 and 40 %

Application of Plate Heat Exchanger for Low Temperature Lift Heat Pump Systems

In this thesis I investigated the energy saving potential of a low temperature lift heat pump (LTLHP) such as water source heat pump (WSHP), and studied the utilization of a plate heat exchanger (PHE) as the LTLHP evaporator. Due to the facility limitation, I only tested and studied the evaporator for LTLHP. Since the LTLHP requires a large water flow rate, its heat source liquid-to-refrigerant PHE is operated at lower refrigerant mass flux than typical applications. I varied the vapor quality, heat flux, evaporation pressure, and refrigerant mass flux to provide unique heat transfer characteristics, and I studied their effects on evaporation heat transfer. Based on the collected data, I concluded that at a low mass flux range, evaporation heat transfer is dominated by nucleate boiling, and convective boiling has mall influence. In addition, I carried out a simulation to compare the performance of WSHP with air source heat pump (ASHP)

Heat Transfer and Pressure Drop during Evaporation of R134a in Microchannel Tubes

The paper presents result for heat transfer and pressure drop in evaporation of R134a in microchannel tubes conducted in a facility with a 6 m long tube, modified to provide realistic situations for refrigerant blends with even the highest glide. The concept of the experimental facility is to measure heat transfer coefficient and pressure drop on the refrigerant side in condensation and evaporation with or without oil with heat exchanger in mind. The auto-controlled test line has 6 test sections for testing and 5 conditioning sections to preset the inlet quality of each test section. This facility provides data in the complete process of evaporation (quality from 0 to 1) or condensation (quality from 1 to 0) in a single pass. The secondary fluid in coolant loop for heating or cooling is water. By controlling the inlet water temperature of each test section, both constant wall temperature and constant heat flux conditions or anything in-between can be achieved. The tertiary loop is a chiller loop running with glycol/water mixtures to cool the water and refrigerant. First results with R134a in this facility show heat transfer coefficient and pressure drop changes with vapor quality and represent excellent starting point (baseline) for explorations of mixtures of low pressure and low GWP refrigerants that are replacements for R410A.

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