Compact/micro heat exchangers – Their role in heat pumping equipment

This paper was presented at the 2nd Micro and Nano Flows Conference (MNF2009), which was held at Brunel University, West London, UK. The conference was organised by Brunel University and supported by the Institution of Mechanical Engineers, IPEM, the Italian Union of Thermofluid dynamics, the Process Intensification Network, HEXAG - the Heat Exchange Action Group and the Institute of Mathematics and its Applications.Compact and micro-heat exchangers have many advantages over their larger counterparts, particularly when used to handle clean fluid streams, either single- or two-phase. Probably the most exciting feature of such heat exchangers is their ability to operate with close approach temperatures, leading to high effectiveness. This can be particularly beneficial when the exchangers are used in power-producing or power-consuming systems, where the improved heat exchanger effectiveness can be immediately realised in higher power outputs or reduced power consumption. In the case of heat pumping equipment – the most common examples being air-water or air-air vapour compression cycle heat pumps for domestic heating – this manifests itself in an increased Coefficient of Performance (COP) that reduces CO2 emissions due to a lower energy input needed to drive the compressor. This paper discusses some of the work carried out in five countries, Austria, Japan, Sweden, USA and the UK, within the IEA Heat Pump Implementing Agreement Annex 33 to identify the heat exchangers that can most benefit heat pump cycles, with a strong emphasis on micro-channel heat transfer. It also presents data on other research relevant to the subject, with an emphasis on the ‘micro’ size range

Regulation of Nanorefrigerant Use:A Proactive Measure Against Possible Undesirable Health and Environmental Implications

Recent research studies have shown the very high potentials nanotechnology application has in the refrigeration and air conditioning industry for improving the thermophysical properties of refrigerants and lubricants hence leading to systems with higher COP and much reduced size. The successes achieved with nanorefrigerants are connected to the improved technology for the preparation of nanofluids that has led to achieving more homogeneous and stable mixture of the base fluid and nanoparticle. However, available literatures point to the fact that nanoparticles can find its way into the human cells and as such is a potential health risk for humans and animals. At present, emphasis appears to concentrate more on the technological successes achieved with nanorefrigerants, like improved thermal conductivity, nucleate boiling and boiling heat transfer coefficient without factoring in much the potential health implications of adopting it. This paper therefore looked at the potential health consequences of adopting nanotechnology and called for an early regulatory framework to guard against any unforeseen health issues. Areas of future research were also suggested

Magnetic Field Enhancement in Ammonia-Water Absorption Refrigeration Systems

Absorption enhancement has been considered as an effective way of improving coefficient of performance (COP) of refrigeration systems and magnetic enhancement is one of these methods. A model of magnetic field enhancement in ammonia-water absorption systems is presented in this paper. A numerical model using finite difference scheme was developed based on the conservation equations and mass transport relationship. Macroscopic magnetic field force was introduced in the momentum equation. The model was validated using data obtained from the literature. Changes in the physical properties of ammonia solution while absorbing both in the direction of falling film and across its thickness were investigated. The magnetic field was found to have some positive effect on the ammonia-water falling film absorption. The results indicate that absorption performance enhancement increased with magnetic intensity. The COP of simple ammonia solution absorption refrigeration system increased by 1.9% and 3.6% for magnetic induction of 1.4 and 3.0 Tesla respectivel

Assessment of alumina nano fluid as a coolant in double pipe gas cooler for trans-critical CO2 refrigeration cycle

In this study, the performance of an alumina nanofluid cooled double pipe gas cooler fortrans-critical C02 refrigeration cycle is theoretically compared to that of water cooled gas cooler. Equal pumping power comparison criterion is adopted besides conventional equal Reynolds number comparison base. Nanofluid is loaded with 0.5%, 1.5% and 2.5% of particle volume fraction under turbulent flow conditions. Drastic variation of thermal and transport properties of CO2 in the vicinity of pseudo criticai temperature is taken care of by employing an appropriate discretization technique. Effect of gas cooler pressure, Reynolds number, pumping power and nanoparticle volume fraction on COP of refrigeration system, gas cooler overall conductance, effectiveness and its capacity has been studied. Results indicate that at equal Reynolds number comparison, performance for alumina nanofluid cooled system is better than that of water cooled system. On the other hand, at equal pumping power comparison basis, the performance of water cooled system is superior. Even at equal mass flow rate comparison criterion, the performance of nanofluid cooled system degrades with increase in particle volume fraction. This study is expected to help to assess the nano fluid as a coolant before expensive experimentation

Cold storage condensation heat recovery system with a novel composite phase change material

© 2016 Elsevier Ltd. Using condensation heat from cold storage refrigeration systems to provide heat for domestic hot water preparation and industrial hot water supply promotes energy conservation. However, few studies have investigated cold storage condensation heat recovery using phase change materials (PCMs). In this study, a cold storage condensation heat recovery system that uses PCMs has been designed and analysed. According to the principle of energy cascade recycling, different operation modes could be effectively switched to recycle condensation heat. Furthermore, a novel and suitable phase change composite material is developed for cold storage condensation heat recovery, which has a relatively large latent heat, high thermal conductivity, and an appropriate phase change temperature (i.e. 80 °C). With carnauba wax (CW) as the PCM and expanded graphite (EG) as the additive, a composite was developed with an optimal mass ratio of CW:EG = 10:1. The thermal and physical properties and the interior structure of the composite were then investigated using a scanning electron microscope (SEM), thermal constants analyser (Hot Disk), differential scanning calorimeter (DSC), and Fourier transform infrared spectrometer (FT-IR). Furthermore, experiments on the melting and solidification processes and accelerated thermal cycling were also conducted. It was found that at the optimal mass ratio of 10:1, the temperatures of the CW/EG composite in the melting and solidification processes were 81.98 °C and 80.43 °C, respectively, while the corresponding latent heats were 150.9 J/g and 142.6 J/g, respectively. During both processes, CW could retain its original worm-like structure after being completely adsorbed by EG. Compared to only CW, the melting and solidification time of the CW/EG composite were reduced by 81.7% and 55.3%, respectively, while its thermal conductivity was 16.4 times higher. After 1000 runs of accelerated thermal cycling, the endothermic/exothermic phase change temperatures of CW and the CW/EG composite increased by only 0.42%/0.42% and 0.23%/0.27%, respectively, while their endothermic/exothermic latent heats decreased by 4.96%/4.78% and 2.05%/3.44%, respectively. These results indicate that both CW and the CW/EG composite have excellent thermal reliability, while the CW/EG composite exhibits a slightly better performance. Finally, the experiments show that the CW/EG composite has desirable thermal and physical properties such as high thermal conductivity and reliability; Hence, it has good potenti al as a material for facilitating condensation heat recovery from cold storage refrigeration systems

Design of an environmentally friendly refrigeration laboratory based on cooling capacity calculation for graduate students

Lower global warming potential (GWP) refrigerants must be used in refrigeration education to integrate the environmentally responsible engineering principles in class. However, most of the refrigeration educational laboratories are still using hydrofluorocarbons (HFCs) as working fluids, which are considered as greenhouse gases. This paper shows the procedure to adapt the new refrigerant R513A in a refrigeration system used for a cooling capacity educational laboratory. First, the paper describes the organization of the laboratory session, and the characteristics of the different methods of cooling capacity calculation taught to the master’s degree students. Then, the benefits of including new sensors in the experimental setup to obtain more accurate results are explained. Later, accurate new graphics and an equation to calculate the R513A cooling capacity are provided. Finally, the educational aspects worked with the students in this session, and each cooling capacity method are assessed. The procedure explained in this paper can be used as a guide for introducing lower GWP refrigerants in similar educational refrigeration laboratories

Experimental performance of LPG refrigerant charges with varied concentration of TiO2 nano-lubricants in a domestic refrigerator

This article present an experimental investigation of varied mass charges of Liquefied Petroleum Gas (40 g, 50 g, 60 g and 70 g) enhanced with varied TiO2 nanoparticle/mineral oil concentrations (0.2 g/L, 0.4 g/L and 0.6 g/L nanolubricants) in a R134a compressor of a domestic refrigerator. Performance tests investigated at steady state included: pull down time, power consumption, compressor power input, cooling capacity and coefficient of performance (COP). Analysis was based on temperature and pressure readings obtained from appropriate gauges attached to the test rig. Refrigerant property characteristics were obtained using Ref-Prop NIST 9.0 software. Results obtained showed almost equal evaporator air temperatures and reduction in power consumption for all tested nano-lubricant concentrations except at 70 g charge of LPG using 0.6 g/L nano-lubricant. Furthermore, the lowest compressor power input was found to be 21 W and obtained using 70 g of LPG with either of 0.2 g/L or 0.4 g/L nano-lubricants. At 70 g of LPG using 0.6 g/L concentration of nano-lubricant, highest cooling capacity index of 65 W was obtained while the highest COP of 2.8 was obtained with 40 g charge of LPG using 0.4 g/L concentration of nanolubricant. In conclusion, LPG-TiO2 nano-lubricant mixture works safely and efficiently in domestic refrigerators without modification of capillary tube length, but requires adequate optimization

Experimental Study of Slug Flow for Condensation in a Square Cross-Section Micro-Channel at Low Mass Velocities

This paper was presented at the 4th Micro and Nano Flows Conference (MNF2014), which was held at University College, London, UK. The conference was organised by Brunel University and supported by the Italian Union of Thermofluiddynamics, IPEM, the Process Intensification Network, the Institution of Mechanical Engineers, the Heat Transfer Society, HEXAG - the Heat Exchange Action Group, and the Energy Institute, ASME Press, LCN London Centre for Nanotechnology, UCL University College London, UCL Engineering, the International NanoScience Community, www.nanopaprika.eu.In this paper, condensation flows in a cross-flow air-cooled micro-condenser were investigated for mass velocities (representing the mass flow rates over the micro-condenser cross-section area) lower than 12 kg.m (−2).s(−1), with n-pentane used as the working fluid. This micro-condenser consisted of a transparent square cross-section micro-channel placed horizontally, having inner and outer edges of 553 and 675 μm, respectively, and a real length exposed to the coolant of 196 mm. One of the specificities of the experimental bench was the choice of the air as a coolant so that the external heat transfer is limiting. Three main flow zones were identified: annular zone, intermittent (i.e. elongated bubbles or slug) zone and spherical bubbles zone. A specific experimental procedure based on bubbles tracking was developed in order to determine the hydraulic and thermal parameters in the intermittent zone. The mean displacement and condensation velocities of the elongated bubbles were determined according to their mean length for different mass velocities of the n-pentane. Besides, the mean latent heat flux density released by the condensation of the elongated bubbles was determined according to their mean surface for different mass velocities of the n-pentane, and compared to the imposed heat flux density

Influence of metallic porous microlayer on pressure drop and heat transfer of stainless steel plate heat exchanger

This paper was presented at the 4th Micro and Nano Flows Conference (MNF2014), which was held at University College, London, UK. The conference was organised by Brunel University and supported by the Italian Union of Thermofluiddynamics, IPEM, the Process Intensification Network, the Institution of Mechanical Engineers, the Heat Transfer Society, HEXAG - the Heat Exchange Action Group, and the Energy Institute, ASME Press, LCN London Centre for Nanotechnology, UCL University College London, UCL Engineering, the International NanoScience Community, www.nanopaprika.eu.The experimental analysis of passive heat transfer intensification in the case of plate heat exchanger has been carried out. On the heat transfer surface of heat exchanger the metallic porous layer was created. The experiment was accomplished in two stages. In the first stage the commercial stainless steel gasketed plate heat exchanger was investigated, while in the second one – the identical heat exchanger but with the modified heat transfer surface. The direct comparison of thermal and flow characteristics between both devices was possible due to the assurance of equivalent conditions during the experiment. Equivalent conditions mean the same volumetric flow rates and the same media temperatures at the inlet of heat exchangers in the corresponding measurement series. Experimental data were collected for the single-phase convective heat transfer in the water-ethanol configuration. The heat transfer coefficients were determined using the Wilson method