No description available.

No description available.

This experimental study explores the heat transfer from heated bare and finned tubular surfaces to particulates in packed bed cross flow. The results from this experiment will be used to help select the type of particulates that will be used. Additionally, these results will assist in estimating heat transfer in prototype and commercial particle to fluid heat exchangers (PFHX). This research is part of larger effort in the use of particulates in concentrating solar power technology. These solid particles are heated by concentrated sunlight to very high temperatures at which they are a suitable heat source for various thermal power and thermochemical cycles. Furthermore, one of the advantages of this concept is the ability to store thermal energy in the solid particles at relatively low cost. However, an important feature of any Particle Heat Receiver (PHR) system is the PFHX, which is the interface between the solar energy system and the thermal power or chemical system. In order to create this system material data is needed for the design and optimization of this PFHX. The paper focuses on the heat transfer properties of particulates to solid surfaces under plug flow conditions. The particulates will be evaluated for three grain sizes of sand and two grain sizes of proppants. These two materials will be tested at one, five and ten millimeters per second in order to see how the various flow rates, which will be required for different loads, will affect the heat transfer coefficient. Finally the heat transfer coefficient will also be evaluated for both finned and non-finned heat exchangers to see the effect that changes in the surface geometry and surface area have on the heat transfer coefficient. The heat transfer coefficient will help determine the appropriate material that will be used in the PHR system.

Concern with global climate change has led to the interest in the use of natural refrigerants, such as carbon dioxide, as replacements in heat pump systems. When operating in a transcritical cycle, carbon dioxide heat pumps are well suited for use in high temperature water heating heat pumps. In this work, four systems are analyzed: with and without the use of a suction line heat exchanger, and two water heating schemes. These two schemes involve the heating of water to its desired temperature at a low water flow rate in a single pass through the heat pump, and the heating of water at a high water flow rate, but requiring multiple passes. The performance and resulting heat exchanger size of these four systems is analyzed through the development of an overall system model. This system model uses component-level models that were developed based upon heat exchanger geometry and subsequently validated through experimental testing on a test facility developed for this purpose.

The large proportion of energy used in the built environment has made improving energy efficiency in buildings, in particular their heating, ventilation, and air conditioning (HVAC) systems, a policy objective for reducing energy consumption and CO2 emissions nationally and internationally. Ground source heat pump (GSHP) systems, due to their high coefficient of performance (COP) and low CO2 emissions are consequently, receiving increasing attention. This work is concerned with the modelling of borehole heat exchangers (BHEs), the commonest form of ground heat exchangers found in GSHP systems. Their careful design is critical to both the short timescale and long timescale performance of geothermal heat pump systems. Unlike conventional components of HVAC systems, BHEs cannot be designed on the basis of peak load data but require 3 application of dynamic thermal models that are able to take account of the heat transfer inside the borehole as well as the surrounding ground. The finite volume method has been applied to develop a dynamic three-dimensional (3D) model for a single BHE and BHE arrays. The multi-block boundary fitted structured mesh used in this model allows the complex geometries around the pipes in BHEs and the surrounding ground around the borehole to be represented exactly. The transport of the fluid circulating along the pipe loop has been simulated explicitly in this model. The ground underneath the borehole has also been represented in this model. Validation of the 3D model has been carried out by reference to analytical models of borehole thermal resistance and fluid transport in pipes, as well as experimental data. In this work, the 3D numerical model has been applied to investigate the three-dimensional characteristics of heat transfer in and around a BHE at both short and long timescales. By implementing a two-dimensional (2D) model using the same numerical method and comparing the simulation results from the 3D and 2D models, the most significant three-dimensional effects have been identified and quantified. The findings have highlighted some of the limitations of 2D models, and based on the findings, methods to improve the accuracy of a 2D model have been suggested and validated. Furthermore, the 3D and 2D finite volume models have been applied to simulate an integrated GSHP system and their effects on overall system performance predictions have been investigated. The 3D numerical model has also been applied to examine thermal interactions within BHE arrays and to evaluate the assumptions of the line source model and their implications in the analysis of thermal response test data.

Detta examensarbete utfördes på uppdrag av Umeå Kommun. Uppgiften bestod av att utvärdera dagens användning av en borrhålsbrunn samt undersöka vad som är det optimala användningsområdet för borrhålsbrunnen. Borrhålsbrunnen används i dagsläget för att förvärma samt kyla utomhusluft in till kontorsbyggnaden Kubens ventilationsaggregat. Ventilationens förvärmning består av två delar, en markkanal och ett geoenergibatteri. Viktiga parametrar hos förvärmningen analyserades med hjälp av mätvärden för temperatur och flöde. Resultaten jämfördes därefter med alternativet att använda borrhålsbrunnen tillsammans med en värmepump. Då borrhålsbrunnens kapacitet inte var tillräcklig för att klara byggnadens hela uppvärmningsbehov, undersöktes det hur en värmepump skulle kunna köras i kombination med fjärrvärme. Två driftstrategier, Bas och Kapatoppar, undersöktes. Bas leverar en basproduktion under hela vinterperioden medan Kapatoppar startar vid -6°C för att sänka effekttoppar. Utvärderingen av förvärmningen visade att geoenergibatteriet är mer kompatibelt med ventilationsaggregat av VAV-typ (Variable Air Volume) än vad markkanalen är. Däremot är inte förvärmning i kombination med roterande värmeväxlare ett bra koncept för byggnaden som den används idag. På grund av att förvärmningen är placerad före den roterande värmeväxlaren så är bara 15-20 % av förvärmningseffekterna energibesparande. Detta kombinerat med en optimerad drift av aggregatet sett till tidsstyrning och behovsstyrning av flöde gör att förvärmningen har en liten påverkan både på byggnadens maximala effektbehov och totala energibehov. Att köra värmepump i kombination med fjärrvärme för uppvärmning var ekonomiskt lönsamt främst på grund av att värmepumpen kunde kapa byggnadens effektoppar. Kostnaden för storleken på den abonnerade effekten uppgår idag till drygt 40 % av den totala fjärrvärmekostnaden. Dagens relation mellan el- och fjärrvärmepriser bidrog naturligtvis också till att värmepumpsalternativet var lönsamt. För de undersökta förutsättningarna så blev paybacktiden för en värmepump med den lönsammaste driftstrategin, Bas, 4,4 år.

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Due to the high performance of electronic components, the heat generation is increasing dramatically. Heat dissipation becomes a significant issue in efficiency promotion and stable operation. Microchannels are of current interest for use in heat exchangers where very high heat transfer performance is desired. Microchannels provide high heat transfer coefficients because of their small hydraulic diameters. In this study, the design and experimental investigation of fluid flow and heat transfer in a microchannel heat exchanger is conducted. Water and air are used as the working fluids and flowed through microchannels. The heat exchanger has been designed with 6 rows of microchannels for water flow and 7 rows of microchannels for forced flow of air. The heights of the microchannels are 4 mm and 10 mm respectively for water and air flows. Microchannels are brazed to form the heat exchanger. For forced convection cooling with air, a military fan is used. A constant heat source has been specifically designed for experiments. Water flow and heat transfer experiments are conducted on the aluminum microchannel heat exchanger. An experimental method of imposing a constant heat flux to water prior to the entrance to the microchannel heat exchanger, to adjust the inlet temperatures is used. v From the data obtained, the rate of heat transfer, effectiveness and various other parameters have been computed and the results have been compared with those from an available commercial heat exchanger. The results indicate that the heat exchanger performs well and provides 681 W of cooling in a volume 677.6 cm3 while the commercial heat exchanger provides 702.5 W of cooling in a volume 2507.5 cm3. In addition, air-side Colburn modulus has been obtained with respect to Reynolds number.

This dissertation provides a detailed description of the research work completed on fin staged heat exchangers. The effects of staging fin on the frosting performance of heat pump evaporators and the whole heat pump system have been studied experimentally and theoretically. Frost degrades the performance of fin-and-tube outdoor coils as well as the whole heat pump system. The objective of the experimental part of this study was to investigate the effects of the staging fin on the frost/defrost performance of heat pump outdoor coils under different operating conditions. To accomplish this objective, a series of frosting tests was conducted on an off-the-shelf heat pump system with five (three two-row and two three-row) evaporators over a range of outdoor temperatures and humidities and a range of airflow rates typical of those found in residential sized heat pumps. Performances of the heat pump unit with baseline or fin staged outdoor coils at either frosting or steady-state test conditions are compared and analyzed. Experimental data showed that for a given tworow heat pump outdoor coil operating at the standard ANSI/ASHRAE 35 °F (1.7 °C) frosting conditions, fin staging increased cycle time and COP. There was a small decrease in peak capacity at lower initial airflow rates. At a lower temperature of 28 °F (2.2 °C), cycle time continued to be enhanced with fin staging, and cyclic COP was within 5% of the base case when fin staging was used. In the second step of this work, an analytical model to simulate the performance of both the baseline and fin staged heat pump coils under frosting conditions was developed based on fundamental heat and mass transfer principles. The transient performance of the frosted evaporator was analyzed with the quasi-steady state approach. The section-by-section evaluation scheme was combined with the tubeby- tube approach to model the mass transfer process in the frost formation module. The two-dimensional fin surface was divided into a number of parallel non-overlapping sections. Each of the sections was the calculation unit for the mass transfer. Methods for calculating the airside heat transfer coefficient and friction factor were developed and applied to the simulation model of the fin staged coil. To verify the validity of the frosted evaporator model, the frosting performance of three two-row coils at the same test conditions was simulated and compared with experimental data. The frosted evaporator model appeared to provide satisfactory simulation of the fin-and-tube heat exchanger during the frost buildup process. Comparisons with the test data indicated that the model could capture the trends of the coil capacity, pressure drop, airflow and frost growth. The model also provided a variety of other simulation results including frost mass accumulation, air velocity inside coil, air and refrigerant outlet state, and so on. Overall, the numerical results were in reasonable agreement with the test data under different frosting operation conditions.

Power generation through decentralized small scale CHP would facilitate the use of biomass as an energy source, with the externally fired gas turbine (EFGT) being a promising technology due to its high electrical efficiency. In an EFGT hot flue gases are heat-exchanged with an air cycle, driving the turbine. The operation requires higher flue gas temperatures than other technologies, for example steam turbines, to achieve optimal performance.  The operating conditions subjects the high temperature heat exchanger (HT-HE) to both physical and chemical stress, with the corrosion related issues yet to be solved. Problems concerning deposit formation and corrosion, on for example super heaters and heat exchangers, when firing biomass are important issues even in commercially available technologies, where the choice of fuel and fuel additives together with component design and choice of material plays important roles in order to minimize the problems. The significantly higher temperatures of the heat transferring surfaces for an EFGT entails combustion deposit related problems less studied. The evaluation of turbine control, deposit formation and corrosion as well as design of the HT-HE and system integration will enable the development of the EFGT technology for applications with small- and medium-size biomass combustion. In this work four potential HT-HE alloys of various grades have been evaluated with respect to corrosion resistance, when exposed to alkali salts and salt mixtures in the KCl-K2CO3-K2SO4 system. The exposures were done in a tube furnace during 24 h for each experiment at four temperature levels between 700–1000oC. Morphological and elemental analysis of the alloy surface and corrosion layers was performed with SEM-EDS. The presence of KCl in the salt caused the most severe corrosion attacks while the corrosion attacks of the pure sulfate and carbonate were more modest. Significant differences between the four materials were observed. X20 experienced severe corrosion, with corrosion scale formation in most cases. The KCl-containing salts caused 253MA to form corrosion scales at all temperatures, while the corrosion resistance to other salts was fairly good. Inconel 600 had the second best overall corrosion resistance. However, it should be pointed out that in some cases the alloy was surpassed by 253MA. Kanthal showed the best overall performance, with limited corrosion scale formation and surprisingly high corrosion resistance to the KCl-containing ternary salt mixture at 900°C and 1000°C.