M.Ing. / Dry air cooled heat exchangers form a vital part of industrial heat transfer systems, especially in countries where the supply and availability of clean cooling water is limited. Headerboxes are rectangular pressure vessels that act as the inlet distribution and outlet collection devices. As rectangular pressure vessels, headerboxes are subject to design codes such as ASME (ASME Section VIII, Division 1, 2007). Unfortunately ASME (ASME Section VIII, Division 1, 2007) offers no guidance on how to allow for the effect of external loads applied to the headerbox through the vessel’s nozzles. This creates a difficult situation, since vessel designers are mandated by ASME to consider the effects of nozzle loads by American Petroleum Institute standard 661 (API, 2006). The aim of this project was therefore to develop a closed form design methodology that accurately predicts the stresses inside a headerbox that is subject to external loadings as well as internal pressure. After extensive research it was decided that the only viable approach would be to extend ASME’s rigid frame theory. This was done, and a new set of equations describing the stress distribution inside a headerbox were derived. These equations were then tested using 2D Finite Element Analysis (FEA) to determine whether they represented the reality of the 2D model they described. It was found that the equations were accurate enough in 2D and the next step was to test the model experimentally and using full 3D FEA. A local manufacturer of air cooled heat exchangers was approached and they helped design an experimental specimen and agreed to fund its construction. Unfortunately, due to time constraints, it was not possible to build and test the specimen experimentally. The specimen geometry was then analysed using the Abaqus (Dassault Systѐmes Simulia Corp., 2010) FEA package. The 3D FEA analysis considered several different load cases. After carefully analysing the results it was seen that the rigid frame model could make useful qualitative statements about the effects of the nozzle loads, but it performed poorly as a quantitative prediction method. However, since the effects of the nozzle loads are generally quite small it is possible that, with appropriate safety factors, the rigid frame model could be used as a conservative design methodology. The usefulness of a commonly used empirical guideline was also examined. This project is far from conclusive and much more work is required to fully examine the usefulness of rigid frame theory. That being said, this project has made important steps towards a more complete understanding of rectangular pressure vessels and has shown possible ways forward.

M.Ing. / The research in this thesis focuses on the theoretical thermal modelling and design of a Latent Heat Storage system (LHS) for an absorption refrigeration machine. A shell-and-tube latent heat storage exchanger retaining any excess solar thermal energy is selected. Here, solar thermal energy supplied by a collector is transferred to and stored by the LHS. During low insolation, stored thermal energy is transferred by a Heat Transfer Fluid (HTF) into the generator, a component of an Ammonia Absorption Refrigerator (AAAR), to ensure efficiency of the cooling cycle. The shell-and-tube LHS contains Phase Change Material (PCM) which fills space outside the tube heat exchangers. The HTF flowing through the tubes exchanges thermal energy with the PCM. The selection of a suitable PCM for a LHS is based on several factors. A primary criterion for an efficient, reliable storage unit is the correct melting point of the PCM at a desired operating temperature of the heating application. An analytical model describing both the freezing process in the PCM and increased HTF temperature in the tube heat exchangers is investigated. The model is developed using energy balance equations. It is solved in terms of dimensionless parameters. The thermal resistance of the tube heat exchangers is considered for this model. From the result of the analytical model, the design approach to size the LHS is provided and the different steps are given in order to determine the volume, mass, number of tube heat exchangers, inner and outer radius of the tube heat exchangers and other parameters of the LHS. The dimensions of LHS are given as a function of a storage period, PCM properties, HTF properties, inner and outer radius of the tube heat exchangers, material of construction of the tube heat exchangers and the nature of load on the heating process. Simulations from the analytical model developed are provided for the output thermal parameters of the storage system. These thermal parameters of the shell-and-tube latent exchanger are given in terms of the HTF outlet temperature, the front solidification of the PCM and the heat transfer rate during the solidification process of the PCM. A case study to demonstrate the application of the design approach with respect to the size shell-and-tube latent heat exchanger is provided.The integration of the tube heat exchangers thermal conductivity in the modelling of the LHS resulted in an increase of 2% in mass of the storage material compared to an analytical model neglecting the thermal conductivity of the tube heat exchangers. The results of the model developed compared well with the results obtained from other analytical models at similar operating conditions.

In recent years much attention has been devoted to the development of efficient heat exchangers. The regenerative or moving matrix heat exchanger is one type which has been considered. Most regenerative heat exchangers use one of the conventional types of high area matrices mounted in a drum or disk in such a manner that it can be moved in a continuous cycle from a cold fluid stream to a hot fluid stream and back again. A rather different approach to the design of a regenerative heat exchanger which has been patented in this country is a set of flexible moving bands which carry heat energy from one fluid stream to another in a continuous manner. Such a regenerator is illustrated in Figure 1. This is essentially a parallel plate heat exchanger with a variation in the temperature of the plates and the fluid in the direction of the length of the plate.

No description available.

A computer model of a fin and tube air-to-air heat exchanger is presented. The model incorporates a computational scheme to account for latent effects due to small amounts of moisture in one or both fluid streams. A testing program is described which was performed in order to mathematically characterize the heat transfer and pressure drop relationships of the tube with turbulator used in the heat exchanger. These relationships are included in the computer model. A comparison of the computer model to heat exchanger test data indicates that the computer model may be relied upon to provide design and analysis information. Finally, a parametric study is performed using the computer model in order to explore the characteristics of the heat exchanger and to demonstrate its usefulness to the heat exchanger designer. It is concluded that, in addition to presenting an analysis tool for heat exchanger design, there are several important secondary results. These include: verification of the modelling techniques, the analytical description of the tube with turbulator and, identification of a problem area in the header design.

Yes / Fouling in heat exchangers (HE) is a major problem in industry and accurate prediction of the onset or degree of fouling would be of a huge benefit to the operators. Modelling of the fouling phenomenon however, remains a challenging field of study. Cleaning of heat exchangers, coulpled with the down time, is a financial burden and for industrialized nations and costs can reach to almost 0.25 % of the country’s Gross National Product (Pritchard, 1988).This work presents the development of a dynamic fouling model based on experimental data collected using a laboratory concentric tube heat exchanger handling a saline system. Heat transfer coefficients were obtained from first principles as well as from either the Sieder-Tate or Petukhov-Kirillov correlations modified by Gnielinski depending on the flow regime. The outlet temperatures were calculated using the Effectiveness-NTU method. The dynamic fouling factor was based on the Kern and Seaton fouling model and validation was completed by comparing the experimental outlet temperatures with those predicted by the model. The model predicts the outlet temperatures with an average discrepancy of 1.6 °C and 0.4 °C for the cold and hot streams respectively.

No description available.

Thesis (MEng) -- Stellenbosch University, 1991. / ENGLISH ABSTRACT: In the present study various mathematical models have been developed for the analysis of spray cooled finnned-tube heat exchangers. These models range from simplified models based on the work by Kreid et al. [78KR1], Nakayama et al. [88NA1] and Erens et al. [90ER1] to a more comprehensive model based on the work by Poppe [84P01]. Computer programs were written to evaluate the performance prediction of these models on spray cooled heat exchangers in the vertical air flow mode. Experimental tests were conducted on a four-row finned-tube heat exchanger in a vertical air /water mist flow to verify the mathematical models and it was found that the performance of this heat exchanger could be predicted within 20 per cent using the accurate model. Significant performance enhancement (up to 3.5 times the dry performance) was found by spraying relatively small amounts of water onto the heat exchanger. The spray water massflow rate was found to have a significant effect on the two phase pressure drop across the heat exchanger. The present study indicated certain important factors which have to be taken into consideration when designing spray cooled heat exchanger units. These include the geometry of the finned-tubes, the temperature difference between the process water and the optimum air and the air /spray water ratio. / AFRIKAANSE OPSOMMING: In die tesis is 'n aantal wiskundige modelle ontwikkel wat gebruik kan word vir die analise van sproei verkoelde vinbuis warmte uitruilers. Die wiskundige modelle wissel van eenvoudige modelle wat gebaseer is op die werk van Kreid et al. [78KR1 ], Nakayama et al. [88NA1] en Erens et al. [90ER1] tot 'n omvattende model wat gebaseer is op die werk van Poppe [84 PO 1]. Rekenaar programme is geskryf om die die modelle se voorspellingsvermoe van die kapasiteit van sproei verkoelde warmte uitruilers te evalueer. 'n Eksperimentele ondersoek is gedoen op 'n warmte uitruiler met vier vinbuis rye in 'n toetsseksie met vertikale lugvloei. Die toets resultate is gebruik vir die evaluasie van die wiskundige modelle en het getoon dat die akkurate model se voorspellings binne 20 per sent van die gemete kapasitiet van die uitruiler val. Die eksperirnentele werk het verder aangetoon dat deur klein hoeveelhede water op die uitruiler te sproei, 'n toename in kapasiteit tot 3.5 keer die droe werkverrigting moontlik is. Die twee-fase drukval oor die uitruiler is sterk bei:nvloed deur die sproei water massavloei. Die analitiese navorsmg wat gedoen is met behulp van die rekenaar programme het aangedui dat daar 'n paar belangrike faktore is wat in ag geneem moet word met die ontwerp van sproei verkoelde warmte uitruilers. Die faktore sluit die vinbuis geometrie, die temperatuur verskil tussen die proses water en lug en optimum lug/sproeiwater massavloei verhouding in.

Thermoacoustics is a field of study which includes devices purpose-built to exploit the phenomenal interaction between heat and sound. Thermoacoustics has been demonstrated as an effective technology which can potentially serve a variety of purposes such as cryogenics, cost-effective domestic refrigeration or electricity generation, without adverse environmental impact or commercial drawbacks such as expensive construction or maintenance costs or high part counts. The mechanisms by which thermoacoustic devices operate at low amplitudes have been identified and effective design tools and methods are available, but the precise heat and mass transfer which occurs deep inside the core of thermoacoustic devices at high amplitudes cannot at present be precisely determined experimentally, and to date have been estimated using only relatively simple or one-dimensional computational domains. It is expected that thermoacoustic devices will need to operate at relatively high pressure amplitudes for commercial and practical applications, to achieve power densities similar to competing technologies. Clearly, advancement of these models and the methods used to investigate them will enable a better understanding of the precise heat and mass transfer that occurs within such devices. Previous numerical studies have modelled a ‘thermoacoustic couple’ which consists of a single or several plates (often modelled with zero thickness) and channels within an oscillatory pressure field. In this thesis several improvements to the ‘thermoacoustic couple’ modelspace are introduced and modelled, and compared with published results. Using the commercial CFD software Fluent, a two-dimensional, segregated and second-order implicit numerical model was developed which solves equations for continuity of mass, momentum and energy. These equations were computed using second-order and double-precision discretisation of time, flow variables and energy. A computational domain is presented which is capable of modelling plates of zero or non-zero thickness, is ‘self-resonant’ and able to capture the entrance and exit effects at the stack plate edges. Studies are presented in which the acoustic pressure amplitude, the thickness of the plate (‘blockage ratio’) and the shape of the plate are varied to determine their influence upon the rate of effective heat transfer, flow structure and overall efficiency. The modelling of thermoacoustic couples with finite thickness presented in this thesis demonstrates that the finite thickness produces new results which show significant disturbances to the flow field and changes to the expected rate and distribution of heat flux along the stack plate. Results indicate that the thickness of the plate, t[subscript]s, strongly controls the generation of vortices outside the stack region and perturbs the flow structure and heat flux distribution at the extremities of the plate. Increases in t[subscript]s are also shown to improve the integral of the total heat transfer rate but at the expense of increased entropy generation. Another contribution of this thesis is the study of the effect that leading and trailing edge shapes of stack plates have on the performance of a thermoacoustic couple. In practice, typical parallel or rectangular section stack plates do not have perfectly square edges. The existing literature considers only rectangular or zero-thickness (1-D) plates. Hence a study was performed to evaluate the potential for gains in performance from the use of non-rectangular cross sections, such as rounded, aerofoil or bulbous shaped edges. Consideration of various types of stack plate edges show that performance improvements can be made from certain treatments to the stack plate tips or if possible, stack plate profiles. This thesis also considers the influence of thermophysical properties and phenomena associated with practical thermoacoustic devices to investigate the applicability of the numerical model to experimental outcomes. Comparisons made between results obtained using the numerical model, linear numerical formulations and experimental results suggest that the numerical model allows comparative study of various thermoacoustic systems for design purposes but is not yet of sufficient scope to fully characterise a realistic system and predict absolute levels of performance. However, the presented method of modelling thermoacoustic couples yields increased insight and detail of flow regimes and heat transportation over previous studies. / http://proxy.library.adelaide.edu.au/login?url= http://library.adelaide.edu.au/cgi-bin/Pwebrecon.cgi?BBID=1316904 / Thesis (Ph.D.) -- University of Adelaide, School of Mechanical Engineering, 2008

Thesis (Ph.D.)--Rochester Institute of Technology, 2008. / Typescript. Includes bibliographical references (leaves 170-176).