Computational Fluid Dynamic Studies of Vortex Amplifier Design for the Nuclear Industry—I. Steady-State Conditions

In this study the effects of changes to the geometry of a vortex amplifier are investigated using computational fluid dynamics (CFD) techniques, in the context of glovebox operations for the nuclear industry. These investigations were required because of anomalous behavior identified when, for operational reasons, a long-established vortex amplifier design was reduced in scale. The aims were (i) to simulate both the anomalous back-flow into the glovebox through the vortex amplifier supply ports, and the precessing vortex core in the amplifier outlet, then (ii) to determine which of the various simulated geometries would best alleviate the supply port back-flow anomaly. Various changes to the geometry of the vortex amplifier were proposed; smoke and air tests were then used to identify a subset of these geometries for subsequent simulation using CFD techniques. Having verified the mesh resolution was sufficient to reproduce the required effects, the code was then validated by comparing the results of the steady-state simulations with the experimental data. The problem is challenging in terms of the range of geometrical and dynamic scales encountered, with consequent impact on mesh quality and turbulence modeling. The anomalous nonaxisymmetric reverse flow in the supply ports of the vortex amplifier has been captured and the mixing in both the chamber and the precessing vortex core has also been successfully reproduced. Finally, by simulating changes to the supply ports that could not be reproduced experimentally at an equivalent cost, the geometry most likely to alleviate the back-flow anomaly has been identified

Carbon-based nanofluids and their advances towards heat transfer applications—a review

Nanofluids have opened the doors towards the enhancement of many of today’s existing thermal applications performance. This is because these advanced working fluids exhibit exceptional thermophysical properties, and thus making them excellent candidates for replacing conventional working fluids. On the other hand, nanomaterials of carbon-base were proven throughout the literature to have the highest thermal conductivity among all other types of nanoscaled materials. Therefore, when these materials are homogeneously dispersed in a base fluid, the resulting suspension will theoretically attain orders of magnitude higher effective thermal conductivity than its counterpart. Despite this fact, there are still some challenges that are associated with these types of fluids. The main obstacle is the dispersion stability of the nanomaterials, which can lead the attractive properties of the nanofluid to degrade with time, up to the point where they lose their effectiveness. For such reason, this work has been devoted towards providing a systematic review on nanofluids of carbon-base, precisely; carbon nanotubes, graphene, and nanodiamonds, and their employment in thermal systems commonly used in the energy sectors. Firstly, this work reviews the synthesis approaches of the carbon-based feedstock. Then, it explains the different nanofluids fabrication methods. The dispersion stability is also discussed in terms of measuring techniques, enhancement methods, and its effect on the suspension thermophysical properties. The study summarizes the development in the correlations used to predict the thermophysical properties of the dispersion. Furthermore, it assesses the influence of these advanced working fluids on parabolic trough solar collectors, nuclear reactor systems, and air conditioning and refrigeration systems. Lastly, the current gap in scientific knowledge is provided to set up future research directions

Numerical and Experimental Study of the Influence of Frost Formation and Defrosting on the Performance of Industrial Evaporator Coils

Under the supervision of Professors Gregory F. Nellis, Sanford A. Klein, and Douglas T. Reindl; 288pp.The main objective of an evaporator in any refrigeration system is to extract thermal energy from a conditioned space by recirculating air through a refrigerated coil. However, when an air-cooled evaporator operates at a temperature below the freezing point of water with a coincident entering air dew point temperature that is above the evaporator coil surface temperature, frost will form on the evaporator surface. The presence of frost reduces the performance of an evaporator and the penalty is proportional to the amount of frost that has accumulated. For this reason, the accumulated frost must be periodically removed by the use of a defrost process. A variety of methods are used to remove frost, however, the most widely-used defrost technique in industry is hot gas defrosting (HGD). The HGD technique depends on temporarily converting the evaporator to a condenser by passing hot gas through the coil; the hot gas is usually obtained directly from the compressor discharge. The HGD technique is a simple and effective method to remove frost rapidly, and the additional hardware required for the HGD process is relatively inexpensive to install. However, during the HGD process, a fraction of the energy supplied to the coil is ultimately transferred to the refrigerated in various forms and becomes a parasitic load (latent and sensible) on the refrigerated space. This additional energy added to the space must be extracted by other evaporators within the freezer space (or, if only one evaporator is in the space then the product temperature must rise). Hence, both the frosting and the defrosting processes penalize the efficiency of the cooling system. In this research, the performance of a large scale industrial evaporator operating under frosting conditions is experimentally monitored during both cooling mode (which occurs under frosting conditions) and defrost mode. Theoretical models of the evaporator coil during the cooling and the defrosting modes have been developed and validated using the experimental data. The degradation of the performance of the evaporator during the cooling mode and the parasitic heat load associated with the defrost mode are presented. The two models are used to optimize the net cooling by minimizing all the penalties associated with running the refrigeration system. Guidelines relative to the most energy efficient operation of industrial refrigeration systems are presented.Sponsored by Kuwait University

Experimental evaluation for potential drop in refrigerants under high-ambient conditions

Compliance with the Kyoto Protocol and United Nations Sustainable Development Goals requires nations to lower greenhouse gas emissions. Air conditioners, as contributors to direct and indirect emissions of greenhouse gases, can play a major role in limiting global warming to 1.5 °C above pre-industrial levels. Nations classified as Article Five parties, including 147 nations, are required to completely phase out Hydrochlorofluorocarbons including R-22 in 2040. Some of these nations reside in extremely hot climates, and new refrigerants perform poorly under hot weather conditions. While the time is ticking to the phaseout, transitional solutions can be used to move away from R-22 to facilitate the phase-down, especially for units that are still in service.In this paper, a comparative experimental assessment of R453A and R458A as drop-in replacements for R22 is presented. This study aims to evaluate the performance of the two refrigerants to assess their suitability for countries residing in hot climates during the transitional phase-out and phase-down of refrigerants. R22 is used as the baseline refrigerant because it is widely used in developing nations and performs well under high ambient temperatures, reaching as high as 50 °C. Six ambient temperatures (35 °C, 40 °C, 46 °C, 48 °C, 50 °C, and 52 °C) were tested to evaluate the performance of a concealed ducted split air-conditioning unit. The unit rated cooling capacity is 10.39 kW at 48 °C. The measured cooling capacity in comparison to that of R22 was between 87 % and 96 % and 88 % to 96 % for R453A and R458A, respectively, over the tested range of temperatures. The coefficient of performance compared to the baseline showed degradation of 6 %–17 % and 3–10 % for R435A and R458A, respectively. The compression ratio increased by 9 % to 15 % on average for both tested alternative refrigerants over the tested temperature range. It is concluded that R458A performs well as a drop-in alternative refrigerant to R22 and surpasses that of R453A, as it showed slightly less deviation in coefficient of performance

Modelling, Analysis and Entropy Generation Minimization of Al₂O₃-Ethylene Glycol Nanofluid Convective Flow inside a Tube

Entropy generation is always a matter of concern in a heat transfer system. It denotes the amount of energy lost as a result of irreversibility. As a result, it must be reduced. The present work considers an investigation on the turbulent forced convective heat transfer and entropy generation of Al2O3-Ethylene glycol (EG) nanofluid inside a circular tube subjected to constant wall temperature. The study is focused on the development of an analytical framework by using mathematical models to simulate the characteristics of nanofluids in the as-mentioned thermal system. The simulated result is validated using published data. Further, Genetic algorithm (GA) and DIRECT algorithm are implemented to determine the optimal condition which yields minimum entropy generation. According to the findings, heat transfer increases at a direct proportion to the mass flow, Reynolds number (Re), and volume concentration of nanoparticles. Furthermore, as Re increases, particle concentration should be decreased in order to reduce total entropy generation (TEG) and to improve heat transfer rate of any given particle size. A minimal concentration of nanoparticles is required to reduce TEG when Re is maintained constant. The highest increase in TEG with nanofluids was 2.93 times that of basefluid. The optimum condition for minimum entropy generation is Re = 4000, nanoparticle size = 65 nm, volume concentration = 0.2% and mass flow rate = 0.54 kg/s

Modelling, Analysis and Entropy Generation Minimization of Al2O3-Ethylene Glycol Nanofluid Convective Flow inside a Tube

Entropy generation is always a matter of concern in a heat transfer system. It denotes the amount of energy lost as a result of irreversibility. As a result, it must be reduced. The present work considers an investigation on the turbulent forced convective heat transfer and entropy generation of Al2O3-Ethylene glycol (EG) nanofluid inside a circular tube subjected to constant wall temperature. The study is focused on the development of an analytical framework by using mathematical models to simulate the characteristics of nanofluids in the as-mentioned thermal system. The simulated result is validated using published data. Further, Genetic algorithm (GA) and DIRECT algorithm are implemented to determine the optimal condition which yields minimum entropy generation. According to the findings, heat transfer increases at a direct proportion to the mass flow, Reynolds number (Re), and volume concentration of nanoparticles. Furthermore, as Re increases, particle concentration should be decreased in order to reduce total entropy generation (TEG) and to improve heat transfer rate of any given particle size. A minimal concentration of nanoparticles is required to reduce TEG when Re is maintained constant. The highest increase in TEG with nanofluids was 2.93 times that of basefluid. The optimum condition for minimum entropy generation is Re = 4000, nanoparticle size = 65 nm, volume concentration = 0.2% and mass flow rate = 0.54 kg/s

Pool Boiling Amelioration by Aqueous Dispersion of Silica Nanoparticles

Non-metallic oxide nanofluids have recently attracted interest in pool boiling heat transfer (PBHT) studies. Research work on carbon and silica-based nanofluids is now being reported frequently by scholars. The majority of these research studies showed improvement in PBHT performance. The present study reports an investigation on the PBHT characteristics and performance of water-based silica nanofluids in the nucleate boiling region. Sonication-aided stable silica nanofluids with 0.0001, 0.001, 0.01, and 0.1 particle concentrations were prepared. The stability of nanofluids was detected and confirmed via visible light absorbance and zeta potential analyses. The PBHT performance of nanofluids was examined in a customized boiling pool with a flat heating surface. The boiling characteristics, pool boiling heat transfer coefficient (PBHTC), and critical heat flux (CHF) were analyzed. The effects of surface wettability, contact angle, and surface roughness on heat transfer performance were investigated. Bubble diameter and bubble departure frequency were estimated using experimental results. PBHTC and CHF of water have shown an increase due to the nanoparticle inclusion, where they have reached a maximum improvement of ≈1.33 times over that of the base fluid. The surface wettability of nanofluids was also enhanced due to a decrease in boiling surface contact angle from 74.1° to 48.5°. The roughness of the boiling surface was reduced up to 1.5 times compared to the base fluid, which was due to the nanoparticle deposition on the boiling surface. Such deposition reduces the active nucleation sites and increases the thermal resistance between the boiling surface and bulk fluid layer. The presence of the dispersed nanoparticles caused a lower bubble departure frequency by 2.17% and an increase in bubble diameter by 4.48%, which vigorously affects the pool boiling performance