The refrigerant charge reduction in HVAC\R systems is an important issue because it falls within environmental policies regarding refrigerants contributions to the greenhouse effect. A way to move toward charge reduction is to increase the compactness of heat exchangers, which means more complex designs of the evaporators. Nevertheless, while a large amount of studies have has been published on the thermal and hydraulic analysis of flow boiling of refrigerants in horizontal tubes, very little attention has been given to flow boiling in geometries different from straight tubes. This PhD thesis aims at studying the flow boiling characteristics of third generation (HFC) and fourth generation fluids such as HFO-1234yf in geometries which modify the fluid dynamics and two-phase flow with respect to horizontal straight tubes. To achieve this goal, an experimental test facility was specifically designed and built to conduct refrigerant evaporation experiments. This test facility allowed to perform flow regimes visualizations and pressure drop measurements in singularities (such as sudden contractions and return bends). First, two-phase flow regimes visualizations have been carried out using HFO-1234yf, R-134a and R-410A either in straight tubes or in singularities. A qualitative analysis of the flow behavior and also several comparisons to flow pattern prediction methods from the literature were conducted. The second objective of this work was to characterize the flow disturbances caused by singularities such as sudden contractions and return bends, and to study their effects on the hydrodynamic performance (e.g. pressure drop) of refrigerants.Lastly, pressure drop databases for third and fourth generation refrigerants are presented.

Two-phase heat transfer in mini/micro-channels is capable of meeting the high cooling demands of modern high heat flux applications. The phase change process ensures better temperature uniformity and control for local hot spots. Furthermore, these compact channels could be helpful in reducing the required charge and material inventories.Environmental concerns—mainly ozone depletion and global warming—have instigated a search for new alternatives in refrigeration industry. While new compounds are being developed to address stringent legislative demands, natural alternatives are also coming into prominence. A limited number of investigators have reported on thermal performance of such alternatives. The current study is therefore focused on saturated flow boiling heat transfer, pressure drop and dryout characteristics for three low global warming potential (GWP) refrigerants (R152a, R600a and R1234yf) in a vertical mini-channel.In this study experiments were carried out by uniformly heating a test section (stainless steel tube with 1.60 mm inside diameter and 245 mm heated length) at 27 and 32 oC saturation temperature with 50-500 kg/m2s mass velocities. The effects of various parameters of interest (like heat flux, mass flux, system pressure, vapor quality, operating media) on flow boiling heat transfer, frictional pressure drop and dryout characteristics were recorded. R134a, which has been widely used in several applications, is utilized as a reference case for comparison of thermal performance in this study.Experimental results for saturated boiling heat transfer showed strong influence of heat flux and system pressure with insignificant contributions from mass flux and vapor quality. Two phase frictional pressure drop increased with mass flux, vapor quality and with reduced operating pressure. The dryout heat flux remained unaffected with variation in saturation temperature, critical vapor quality in most cases was about 85%. The experimental results (boiling heat transfer, two-phase pressure drop and dryout heat flux) were compared with well-known macro and micro-scale correlations from the literature. / <p>QC 20141124</p>

In recent years carbon dioxide (CO2, R-744)has moved to the foreground as an environmentally friendly alternative to commonly used CFCs and HFCs, which are being phased out due to its high ozone depleting and global warming potentials. R-744 is not only environmentally friendly but due to its unique properties, it is also ideally suited for the use in heat pump water heaters. High cycle efficiencies are achievable even at high hot water temperatures. The high cycle efficiency not only leads to energy and cost savings but also ties in with the drive for implementation of energy saving measures in South Africa. It is therefore paramount to continue development and implementation of R-744 in heat pump water heaters. Optimizing the cycle efficiency is only possible if detailed component simulation models, taking these unique properties of R-744 into account, are available. The purpose of this study therefore was to develop a detail simulation model of a concentric tube-in-tube water-to-refrigerant evaporator, as well as a fin-and-tube air-to-refrigerant evaporator model. Data from the North-West University R-744 heat pump test bench were used to verify the tube-in-tube evaporator simulation model. The discrepancies in the cooling capacity between the simulation and test bench can be attributed to the presence of lubricant in the system.The fin-and-tube model was verified by testing it against the NIST program EVAP-COND (NIST 2010). Overall there was good agreement between the results of the two programs, with EVAP-COND predicting a lower cooling capacity(6% to 14%) and and a higher pressure refrigerant pressure drop (30% to 50%). It was found that both the heat transfer correlation of Jung et al. (1989) and the pressure drop correlation of Choi et al. (1999) are able to predict the experimental values accurately and are valid for use in both the evaporator models developed. To demonstrate the use of the detail evaporator fin-and-tube model, an evaluation of the different tube geometries, commercially available in South Africa, for use with R-744 fin-and-tube evaporators was done. For a fin-and-tube evaporator it was found that the most cost effective option is to use ⅜" (10.05 mm)copper tubes and the least effective is " (12.6 mm) stainless steel tubes. / Thesis (MIng (Mechanical Engineering))--North-West University, Potchefstroom Campus, 2013.

In recent years carbon dioxide (CO2, R-744)has moved to the foreground as an environmentally friendly alternative to commonly used CFCs and HFCs, which are being phased out due to its high ozone depleting and global warming potentials. R-744 is not only environmentally friendly but due to its unique properties, it is also ideally suited for the use in heat pump water heaters. High cycle efficiencies are achievable even at high hot water temperatures. The high cycle efficiency not only leads to energy and cost savings but also ties in with the drive for implementation of energy saving measures in South Africa. It is therefore paramount to continue development and implementation of R-744 in heat pump water heaters. Optimizing the cycle efficiency is only possible if detailed component simulation models, taking these unique properties of R-744 into account, are available. The purpose of this study therefore was to develop a detail simulation model of a concentric tube-in-tube water-to-refrigerant evaporator, as well as a fin-and-tube air-to-refrigerant evaporator model. Data from the North-West University R-744 heat pump test bench were used to verify the tube-in-tube evaporator simulation model. The discrepancies in the cooling capacity between the simulation and test bench can be attributed to the presence of lubricant in the system.The fin-and-tube model was verified by testing it against the NIST program EVAP-COND (NIST 2010). Overall there was good agreement between the results of the two programs, with EVAP-COND predicting a lower cooling capacity(6% to 14%) and and a higher pressure refrigerant pressure drop (30% to 50%). It was found that both the heat transfer correlation of Jung et al. (1989) and the pressure drop correlation of Choi et al. (1999) are able to predict the experimental values accurately and are valid for use in both the evaporator models developed. To demonstrate the use of the detail evaporator fin-and-tube model, an evaluation of the different tube geometries, commercially available in South Africa, for use with R-744 fin-and-tube evaporators was done. For a fin-and-tube evaporator it was found that the most cost effective option is to use ⅜" (10.05 mm)copper tubes and the least effective is " (12.6 mm) stainless steel tubes. / Thesis (MIng (Mechanical Engineering))--North-West University, Potchefstroom Campus, 2013.

No description available.

The purpose of this study was to investigate the velocity distribution through an annular bed packed randomly with equal sized spheres. Extensive research has been conducted on the velocity distribution inside packed beds packed with equal sized spheres, different sized spheres, deformed spheres, cylinders and Raschig-rings. A majority of these experimental and numerical studies focused on the cylindrical packed bed. These studies and numerical models are all confined to the velocity profile once the fluid flow is fully developed. The development of the velocity through the inlet region of the bed and the fluid flow redistribution in the outlet of the bed is thus neglected. The experimental investigation into the velocity distribution down stream of the annular packed bed of the HTTU indicated that the velocity profile was independent of the mass flow rate for a particle Reynolds number range of 439 £ Re £ 3453 . These velocity profiles did not represent the distribution of the axial velocity due to shortcomings associated with the single sensor hot wire anemometry system used to measure the velocity distribution. A numerical investigation, using the RANS CFD code STAR-CCM+®, into the velocity distribution downstream of an explicitly modelled bed of spheres indicated that the axial velocity distribution could be extracted from the experimental velocity profiles by using an adjustment factor of 0.801. This adjusted velocity profile was used in the verification of the implicit bed simulation model. The implicit bed simulation model was developed in STAR-CCM+®. The resistance of the spheres was modelled using the KTA (1981) pressure drop correlation and the structure of the bed was modelled using the porosity correlation proposed by Martin (1978), while the effective viscosity model of Giese et al. (1998), adjusted by a factor of 0.8, was used to model the velocity distribution in the near wall region. It was found that the structure in the inlet region of the bed, where two walls disturb the packing structure, can be modelled as the weighted average of the radial and axial porosity while the structure in the outlet regions can be modelled by letting the radial porosity increase linearly to unity. The basic shape of the velocity profile is established immediately when the fluid enters the bed. The amplitude of the velocity peaks however increase in magnitude until the velocity profile is fully developed at a distance approximately of five sphere diameters from the bed inlet. The profile remains constant throughout the bed until the outlet region of the bed is reached. In the outlet region a significant amount of fluid redistribution is observed. The amplitude of the velocity peaks is reduced and the position of the velocity peaks is shifted inwards towards the centre of the annular region. The fully developed velocity profile, predicted by the simulation model is in good agreement with profiles presented by amongst others Giese et al. (1998). The current model however also offers insight into the development of the profile through the inlet of the bed and the fluid redistribution, which occurs in the outlet region of the bed. / Thesis (M.Ing. (Mechanical Engineering))--North-West University, Potchefstroom Campus, 2010.

An experimental study was conducted in order to investigate the two-phase-flow phenomena in a mini-size, horizontal impacting tee junction. The test section was machined in an acrylic block with a rectangular cross-section of 1.87-mm height × 20-mm width on the inlet and outlet sides. Air-water mixtures at 200 kPa (abs) and room temperature were used as the test fluids. Four flow regimes were identified visually: bubbly, plug, churn, and annular over the ranges of gas and liquid superficial velocities of 0.04 ≤ JG ≤ 10 m/s and 0.02 ≤ JL ≤ 0.7 m/s, respectively, and a flow regime map was developed. The present flow-regime map was compared with several experimental maps. It is thought from those comparisons that the channel height has a more significant role in determining the flow-regime boundaries than the hydraulic diameter. The two-phase fully-developed pressure gradient was measured in the inlet and the outlet sides of the junction for six different inlet conditions and various mass splits at the junction. Comparisons were conducted between the present data and former correlations. The correlations that agreed best with the present data were identified. Five single-phase test sets were performed. In each set of experiments, the pressure distribution was measured for the whole range of the mass split ratio, Wi/W1. The pressure drop at the junction at each value of Wi/W1 was calculated. Values of the pressure-loss coefficient, , were calculated at various Wi/W1 and inlet Reynolds number. The pressure-loss coefficient was strongly dependent on the inlet Reynolds number in the laminar region, while the results for the turbulent region were almost coincident. Numerical simulations of single-phase flow in an impacting tee junction of identical dimensions to that of the present test-section were performed to confirm the results of the experiments. Phase-redistribution experiments were conducted covering all four inlet flow regimes and models were proposed for predicting the experimental data. Good agreement in terms of magnitude and trend was obtained between the present experimental data and the proposed model. New correlations were developed for the single- and two-phase pressure drop in the junction.

The purpose of this study was to investigate the velocity distribution through an annular bed packed randomly with equal sized spheres. Extensive research has been conducted on the velocity distribution inside packed beds packed with equal sized spheres, different sized spheres, deformed spheres, cylinders and Raschig-rings. A majority of these experimental and numerical studies focused on the cylindrical packed bed. These studies and numerical models are all confined to the velocity profile once the fluid flow is fully developed. The development of the velocity through the inlet region of the bed and the fluid flow redistribution in the outlet of the bed is thus neglected. The experimental investigation into the velocity distribution down stream of the annular packed bed of the HTTU indicated that the velocity profile was independent of the mass flow rate for a particle Reynolds number range of 439 £ Re £ 3453 . These velocity profiles did not represent the distribution of the axial velocity due to shortcomings associated with the single sensor hot wire anemometry system used to measure the velocity distribution. A numerical investigation, using the RANS CFD code STAR-CCM+®, into the velocity distribution downstream of an explicitly modelled bed of spheres indicated that the axial velocity distribution could be extracted from the experimental velocity profiles by using an adjustment factor of 0.801. This adjusted velocity profile was used in the verification of the implicit bed simulation model. The implicit bed simulation model was developed in STAR-CCM+®. The resistance of the spheres was modelled using the KTA (1981) pressure drop correlation and the structure of the bed was modelled using the porosity correlation proposed by Martin (1978), while the effective viscosity model of Giese et al. (1998), adjusted by a factor of 0.8, was used to model the velocity distribution in the near wall region. It was found that the structure in the inlet region of the bed, where two walls disturb the packing structure, can be modelled as the weighted average of the radial and axial porosity while the structure in the outlet regions can be modelled by letting the radial porosity increase linearly to unity. The basic shape of the velocity profile is established immediately when the fluid enters the bed. The amplitude of the velocity peaks however increase in magnitude until the velocity profile is fully developed at a distance approximately of five sphere diameters from the bed inlet. The profile remains constant throughout the bed until the outlet region of the bed is reached. In the outlet region a significant amount of fluid redistribution is observed. The amplitude of the velocity peaks is reduced and the position of the velocity peaks is shifted inwards towards the centre of the annular region. The fully developed velocity profile, predicted by the simulation model is in good agreement with profiles presented by amongst others Giese et al. (1998). The current model however also offers insight into the development of the profile through the inlet of the bed and the fluid redistribution, which occurs in the outlet region of the bed. / Thesis (M.Ing. (Mechanical Engineering))--North-West University, Potchefstroom Campus, 2010.

Thesis (MSc (Mathematics))--University of Stellenbosch, 2009. / ENGLISH ABSTRACT: Packed beds are widely used in industry to improve the total contact area between two substances in a multiphase process. The process typically involves forced convection of liquid or gas through either structured or dumped solid packings. Applications of such multiphase processes include mass transfer to catalyst particles forming the packed bed and the adsorption of gases or liquids on the solid packing. An experimental study on the determination of air flow pressure drops over different packingmaterialswas carried out at the Telemark University College in Porsgrunn,Norway. The packed bed consisted of a cylindrical column of diameter 0.072m and height 1.5m, filled with different packingmaterials. Air was pumped vertically upwards through a porous distributor to allow for a uniform inlet pressure. Resulting pressure values were measured at regular height intervals within the bed. Due to the geometric nature of a Raschig ring packing wall effects, namely the combined effects of extra wall shear stress due to the column surface and channelling due to packing adjacent to a solid column surface, were assumed to be negligible. Several mathematical drag models exist for packed beds of granular particles and an important question arises as to whether they can be generalized in a scientific manner to enhance the accuracy of predicting the drag for different kinds of packing materials. Problems with the frequently used Ergun equation, which is based on a tubular model for flow between granules and then being empirically adjusted, will be discussed. Some theoretical models that improve on the Ergun equation and their correlation with experimental work will be discussed. It is shown that a particular pore-scale model, that allows for different geometries and porosities, is superior to the Ergun equation in its predictions. Also important in the advanced models is the fact that it could take into account anomalies such as dead zones where no fluid transport is present and surfaces that do neither contribute to shear stress nor to interstitial form drag. The overall conclusion is that proper modelling of the dynamical situation present in the packing can provide drag models that can be used with confidence in a variety of packed bed applications. / AFRIKAANSE OPSOMMING: Gepakte materiaal strukture word in die industrie gebruik om die kontak area tussen twee stowwe in meervoudige faseprosesse te vergroot. Die proses gaan gewoonlik gepaard met geforseerde konveksie van ’n vloeistof of ’n gas deur gestruktureerde of lukrake soliede gepakte strukture. Toepassings van sulke meervoudige faseprossese sluit onder andere in die massa-oordrag na katalisator partikels wat die gepakte struktuur vorm of die absorpsie van gasse of vloeistowwe op die soliede gepakte elemente. ’n Eksperimentele ondersoek oor die drukval van veskillende gepakte elemente in ’n kolom is gedoen by die Telemark University College in Porsgrunn, Noorweë. Die gepakte struktuur het bestaan uit ’n kolommet ’n diameter van 0.072m en ’n hoogte van 1.5m. Lug is vertikaal opwaarts gepomp deur ’n poreuse plaat wat gesorg het vir ’n benaderde uniforme snelheidsprofiel. Die druk is toe op intervalle deur die poreuse struktuur gemeet. In die studie is die effekte van die eksterne wande, nl. die bydrae van die wand se wrywing en die vorming van kanale langs die kolom wand, as weglaatbaar aanvaar. Daar bestaan baie wiskundige dempingsmodelle vir gepakte strukture wat uit korrels saamgestel is. ’n Belangrike vraag kan dus gevra word, of laasgenoemde modelle veralgemeen kan word op ’n wetenskaplike manier om die demping deur verskillende gepakte strukture akkuraat te kan voorspel. Probleme wat ontstaan het met die wel bekende Ergun vergelyking, wat gebaseer is op ’n kapillêre model en wat toe verder aangepas is deur empiriese resultate van uniforme sfere, sal bespreek word. Teoretiesemodelle wat verbeteringe op die Ergun vergelyking voorstel sal bespreek word en vergelyk word met eksperimentele data. Daar word ook gewys dat ’n spesifieke porie-skaal model, wat aanpasbaar is vir verskillende geometrieë en porositeite, in baie gevalle beter is as die Ergun vergelyking. ’n Ander baie belangrike aspek van gevorderde modelle is die moontlikheid om stagnante gebiede in die gepakte strukture in ag te neem. Laasgenoemde gebiede sal die totale kontak area sowel as die intermediêre vorm demping verlaag. Die gevolgtrekking is dat wanneer deeglike modulering van dinamiese situasies in die industrie gedoen word kan dempings modelle met vertroue op ’n verskeidenheid gepakte strukture toegepas word.

This thesis has developed wetting characteristics of coal dust from Herrin coal seam in Illinois for improved respirable coal and quartz dust (< 10µm) control. Wetting characteristics were assessed by a wettability technique called Fixed Time Wettability (FTW) method which simulates the actual underground wetting environment. Immediate roof and immediate floor materials bulk samples were found to be 100% wettable by water alone, whereas coal seam samples were 55-90% wettable. Two variables: contact time between water droplets and dust particles and, the temperature of the water were found to affect the degree of wettability. FTW was carried out at as a function of water temperature (7.2oC to 40.5oC) and contact time (10 to 25 seconds) which increased wettability 2-18% and 8-32%, respectively. In addition, the wetted and un-wetted samples collected from wettability experiments were subjected to Scanning Electron Microscopy (SEM) and particle size distribution analyses. It was found that particles are irregular in shape in both wetted and un-wetted fractions and some agglomeration of fine particles was observed in a few un-wetted fraction samples. The concept of increased contact time was incorporated in engineering controls by increasing the wet scrubber filter screen inclination (which is a function of the screen surface area). The filter screen inclination was minimizing the pressure drop across it using Computational Fluid Dynamics (CFD) analysis. It was found that 60 degree (with respect to the vertical) would provide minimal pressure drop and should be considered for implementation. Additional research should be conducted on assessing selective wetting of different particle sizes of coal and quartz in airborne dust, developing data on wettability as a function of particle size and implementing the developed concepts such as contact time and temperature in the field for improved dust control.