<p> Heat-exchangers designed and fabricated in accordance with the existing design standards may be susceptible to damage as a result of excessive tube vibrations caused by the shell-side fluid flow. The present investigat ion was undertaken to further our understanding of the vibration behaviour of tube arrays. </p> <p> An experimental facility and techniques have been developed by means of which the major mechanisms that cause flow-induced vibrations in tube arrays due to cross-flow can be produced and properly identified. </p> <p> The experiments were conducted in a low-speed windtunnel having 305 x 305 mm.working section. The tube-bundle was a parallel-triangular tube-array with pitch/diameter = 1.375 . The array was 27 rows deep with 5 tubes in each row . The tubes were designed such that they could be conveniently removed from outside the wind-tunnel, in order to facilitate studying the effect of tube-bundle size on vibration and flow characteristics . Nineteen identical tubes in the middle of the tube-array were movable and specially designed so that natural frequency and damping could be controlled precisely over a range of values. </p> <p> The experiments have verified that deep inside a closely-packed tube-bank the existence of discrete vortex shedding is not a working hypothesis and the response of a tube in a tube-bundle is expected to be a function of Reynolds number and the number of upstream rows of tubes. From the flow-field velocity power-spectra obtained for the array tested and from the available data existing in the literature, it is seen that there is a strong possibility of predicting the dominating frequency in the flow from a universal Strauhal number. For the first time a fluidelastic stability boundary for the array has been derived and it is noticed that the slope of this boundary is significantly different from that derived by other authors from theoretical considerations. </p> / Thesis / Doctor of Philosophy (PhD)

Heat exchangers are integral parts for thermal management and find countless applications in automotive, aerospace, energy, nuclear power plants, HVAC, etc. Due to intensive research & development and technological advancements in manufacturing technologies, there is an increasing rise in demand for high-performance heat exchangers. In the automotive and aerospace industries, heat exchangers are expected to deliver better thermal efficiency and improve the system’s overall functionality in which they are installed by saving space and being lightweight. Additive Manufacturing (AM) is a ground-breaking and promising technology that offers avenues of opportunities to manufacture parts that were almost impossible to be produced with conventional manufacturing and can improve part performance with lightweight and compact designs. Laser-Based Powder Bed Fusion (LPBF), one of the well-known AM techniques, provides freedom to design complex geometries and fabricate them in a layer-by-layer fashion by exposing a high-density laser on a vertically moving powder bed. The study focuses on the application of AM in re-designing heat exchangers under given design requirements using LPBF. It includes exploring Triply Periodic Minimal Surfaces (TPMS) based structures such as gyroid and realizing them as heat exchanger core. Computational gyroid-based heat exchanger core models were designed and analyzed for thermal and fluid dynamics characteristics. A parametric study and analysis based on gyroid TPMS network type, periodic length, thickness, aspect ratio, and functional grading were carried out to optimize heat exchanger performance as per design conditions and validate their manufacturability using LPBF. Successful printable designs were further used to develop and manufacture prototypes. The study concludes with a comparison between additively manufactured gyroid-based design and conventional shell-and-tube design based on the thermal performance from CFD analysis and the weight of prototypes. It was found that the thermal performance from CFD analysis showed an 18.96% improvement, whereas weight was reduced by 14.8% for the gyroid-based design as compared to the conventional shell-and-tube design. / Thesis / Master of Applied Science (MASc)

No description available.

No description available.

No description available.

No description available.

Rising energy demands and the continual push to find more energy efficient technologies have been the impetus for the investigation of waste heat recovery techniques. Diesel engine exhaust heat utilization has the potential to significantly reduce the consumption of fossil fuels and reduce the release of greenhouse gases, because diesel engines are ubiquitous in industry and transportation. The exhaust energy can used to provide refrigeration by implementing an organic Rankine cycle coupled with a vapor-compression cycle. A critical component in this system, and in any waste heat recovery system, is the heat exchanger that extracts the heat from the exhaust. In this study, a cross-flow microchannel heat exchanger was geometrically examined and thermally tested under laboratory conditions. The heat exchanger, referred to as the Heat Recovery Unit (HRU), was designed to transfer diesel exhaust energy to a heat transfer oil. Two methods were developed to measure the geometry of the microchannels. The first was based on image processing of microscope photographs, and the second involved an analysis of profilometer measurements. Both methods revealed that the exhaust channels (air channels) were, on average, smaller in cross-sectional area by 11% when compared to the design. The cross-sectional area of the oil channels were 8% smaller than their design. The hydraulic diameters for both channel geometries were close to their design. Hot air was used to simulate diesel engine exhaust. Thermal testing of the heat exchanger included measurements of heat transfer, effectiveness, air pressure drop, and oil pressure drop. The experimental results for the heat transfer and effectiveness agreed well with the model predictions. However, the measured air pressure drop and oil pressure drop were significantly higher than the model. The discrepancy was attributed to the model's ideal representation of the channel areas. Additionally, since the model did not account for the complex flow path of the oil stream, the measured oil pressure drop was much higher than the predicted pressure drop. The highest duty of the Heat Recovery Unit observed during the experimental tests was 12.3 kW and the highest effectiveness was 97.8%. To examine the flow distribution through the air channels, velocity measurements were collected at the outlet of the Heat Recovery Unit using a hot film anemometer. For unheated air flow, the profile measurements indicated that there was flow maldistribution. A temperature profile was measured and analyzed for a thermally loaded condition. / Graduation date: 2012

Denna studie har undersökt om förvärmning av luft är en bra metod för att undvika frostbildning i motströmsplattvärmeväxlare i luftbehandlingsaggregat. Frostbildning i värmeväxlare för ventilation är ett problem i kalla klimat och sänker den återvunna energimängden när potentialen för energiåtervinning är som högst.   Tidigare forskning i området har utförts för att bland annat: 1) Definiera säkra förhållanden utan frostbildning i värmeväxlare med laminär luftströmning.  2) Jämföra olika frostkontrollstrategier. 3) Analysera frostbildningens egenskaper och effekt på värmeväxlare i ventilation. Att forskningen inte är entydig för vilken frostkontrollstrategi som är bäst belyser vikten av detta arbete.   I en fallstudie kvantifieras empiriska värmeöverföringskoefficienter som används i en analytisk modell som förutspår energianvändningen för luftvärmning under olika driftfall för ett luftbehandlingsaggregat. Värmeöverföringskoefficienterna tas fram genom mätningar i ett luftbehandlingsaggregat lokaliserat i ett flerbostadshus. Mätobjektet har ett installerat värmebatteri före värmeväxlaren vilket värmer inkommande uteluft till konstant lufttemperatur.  Tillverkaren proklamerar att det sker turbulent luftströmning i den studerade värmeväxlaren. Vid olika lufthastigheter ändras värmeöverföringskoefficienten mer vid turbulent strömning än vid laminär strömning. Att olika lufthastigheter har betydlig inverkan på värmeöverföringskoefficienten överensstämmer med resultatet från mätningarna. Effektiviteten av den aktuella styrningen av förvärmaren har granskats och brister har upptäckts. En ny optimerad styrning föreslås för frostfri drift och minimerad energianvändning. Den optimerade styrningen jämförs sedan med en vanlig avfrostningsmetod och utförs med hjälp av flödesbalansstatistik från ett aggregat med sektionsavfrostningsstyrning. Resultatet visar att förvärmning av luft för frostfri drift använder mindre energi än vid sektionsavfrostningsstyrning.  Den föreslagna styrningen innebär att bypass-spjället öppnas då förvärmaren går på full effekt. Detta för att förhindra påfrostning vid extrema utetemperaturer, då förvärmaren inte ger tillräcklig effekt för att undvika påfrostning. Den föreslagna styrningen regleras utifrån daggpunkten, temperaturen på tilluften innan värmeväxlaren och temperaturen på avluften. Den framtagna värmeöverföringskoefficienten i studien tar inte i beaktning ökad koefficient under fuktiga förhållanden då daggpunkten i frånluften är över fryspunkten och det sker kondens. En framtida studie kan hitta värmeöverföringskoefficienter som förutspår yttemperaturen under kondensutfällning då daggpunkten är över 0°C vilket ger medel för att ta fram en ännu mera energieffektiv styrning. Detta kan utföras genom mätning i ett luftbehandlingsaggregat med hög temperaturverkningsgrad placerat i en fuktig miljö. / This study has analysed if preheating of air is a good method to prevent frost formation in a counter flow plate heat exchanger used in Air Handling Units. Frosting in heat exchangers used in AHU-systems is a problem in cold climates and lowers the energy recovery when its potential is the highest.   Previous research has been done to 1) Define safe operating conditions without frost formation in heat exchangers with laminar flow. 2) Compare different frost control strategies. 3) Analyse frost formation properties and its effects on heat exchangers in AHU-systems. That the research is not unambiguous for which frost control method is the best highlights the importance of this work. Empirical heat transfer coefficients are quantified in this study and are used in the creation of an analytical model of a counter flow heat exchanger that predicts the energy use for heating the air at different operating modes. The heat transfer coefficients are produced by measurements in an AHU-systems located in an apartment building. The AHU has an installed air heater before the heat exchanger which heats the outdoor inlet air to constant temperature.   The manufacturer of the AHU-system proclaims turbulent flow in the heat exchanger. Change of airflow has greater impact on the heat transfer coefficients during turbulent flow compared to laminar flow conditions. This is also derived from the results of the measurements. The effectiveness of the current control system is analyzed, and flaws are discovered. A new control system is therefore proposed for frost free operation and higher energy efficiency. The proposed control system is compared to a frequently used frost control system which uses bypass-dampers to redirect the inlet air to the reheater, to let the warm air stream melt formed frost on the warm air side. This comparison is accomplished by analyzing the usage of the bypass-dampers during different outdoor temperatures in a comparable AHU-system that have the mentioned frost control method. The results show that preheating of inlet air for frost free operation uses less energy than usage of the bypass-dampers to melt formed frost on the warm air side. The proposed control system needs the bypass-dampers to be used when the preheater is on full operation to prevent frost formation at extreme outdoor temperatures when the preheating may not be enough. The proposed control system is regulated by the dewpoint and the temperatures of the exhaust air stream and the supply air stream. The derived heat transfer coefficient that is used to predict the plate-temperature take no account for condensation on the warm air side during humid conditions when the dew point is above the freezing point. Future studies can derive the heat transfer coefficient during condensation which will improve the prediction of the plate-temperature. This would make the system more energy efficient during humid air conditions. This can be done by measurements in an AHU-system with high temperature efficiency placed in a humid environment.

Thesis: M.S., Massachusetts Institute of Technology, Department of Mechanical Engineering, 1978 / Includes bibliographical references. / M.S. / M.S. Massachusetts Institute of Technology, Department of Mechanical Engineering

Thesis (MScEng (Mechanical and Mechatronic Engineering))--University of Stellenbosch, 2010. / ENGLISH ABSTRACT: Large air‐cooled heat exchangers and condensers make use of fan arrays to provide cooling. The performance of the fan arrays are however negatively affected by distorted inlet conditions. Given the size of these fans, quantifying exactly what the detrimental effects are is practically impossible. This serves as motivation for developing a scaled multi‐fan testing platform that is able to mimic the behaviour of a full‐scale array but at a more measureable and manageable scale. This investigation was conducted in two parts. A Reynolds number investigation was conducted to determine what effect incorporating Reynolds number effects will have on the scaled fan performance. Computational fluid dynamics (CFD) was used to determine where turbulent transition onset occurs on the full‐scale fan blades and trip wires were then sized and positioned appropriately to recreate the same transition effect. From this initial investigation it was found that the trip wires have an allround negative impact on fan performance but when compensated for by increasing the blade angle, the tripped fan static pressure rise performance is comparable with the non‐tripped fan performance. This suggests that the Reynolds number effects may be ignored for this scaled fan testing investigation. Scaled equivalents of the fans in cooling arrays were tested in a three row multi‐fan array which, using symmetry, represents a typical bay in a full‐scale installation. Various platform heights were simulated and the corresponding decrease in system performance was compared to a model formulated to predict how fan volumetric effectiveness decreases with platform height. The model deviated from the test data at very low platforms heights. A replacement fan for the scaled equivalent fan was tested while installed as the edge fan of a multi‐fan array, the fan worst affected by any inlet disturbance, and was found to perform similarly to the scaled equivalent fan for most platform heights but exceeded the scaled equivalent fan’s performance for the lowest platform height and the most adverse conditions. This proves the effectiveness of the replacement fan as an edge fan. / AFRIKAANSE OPSOMMING: Groot lugverkoelde warmteruilers maak gebruik van waaier matrikse om verkoeling te voorsien. Die verrigting van die waaieropstelling word egter negatief beïnvloed deur versteurde inlaattoestande. Gegewe die grootte van hierdie waaiers is daar geen praktiese metode om die nadelige invloed van die versteurde inlaattoestande te kwantifiseer nie. Hierdie dien as motivering vir die ontwikkeling van ‘n multi‐waaier toetsplatform wat daartoe in staat is om die gedrag van die volskaal opstelling na te boots maar op ‘n baie meer meetbare en hanteerbare skaal. Hierdie ondersoek was uitgevoer in twee dele. ‘n Reynoldsgetal ondersoek was uitgevoer om te bepaal watter impak dit sal hê op die skaalwaaier verrigting indien die Reynolds getaleffekte ingesluit word in die toetswerk. Berekende vloeidinamka (BVD) was gebruik om te bepaal waar turbulente oorgang voorkom op die volskaal waaierlemme en pooitjiedrade was geselekteer en geposisioneer hiervolgens om dieselfde oorgang te herskep. Vanuit hierdie aanvanklike ondersoek was dit gevind dat die pooitjiedrade ‘n algehele afname in verrigting tot gevolg het, maar wanneer dit oorkom word deur die lemhoek op te stel, die gedrag en verrigting van die gepooitjiede waaier soortgelyk is aan die van die nie‐gepooitjiede waaier. Hierdie gedrag stel voor dat die Reynoldsgetalle maar geïgnoreer kan word vir hierdie skaalwaaier toetswerk. Gelykwaardige skaalwaaiers van die wat in bedryf is in volskaal opstellings was getoets in ‘n drie ry multi‐waaier opstelling wat, deur simmetrie, verteenwordigend is van ‘n tipiese straat in ‘n volskaal opstelling. Verskeie platformhoogtes was gesimuleer en die ooreenstemmende afname in stelsel verrigting was vergelyk met ‘n model wat geformuleer is juis om te voorspel hoe die volumetriese effektiwiteit afneem met platformhoogte. Die model wyk af van die toetsdata by baie lae platform hoogtes. ‘n Vervangingswaaier vir die aanvanklike geskalleerde waaier was getoets as ‘n randwaaier, die waaier wat die ergste benadeel word deur versteurde inlaattoetstande, in die multi‐waaier opstelling. Die vervangingswaaier het soortgelyk aan die aanvanklike waaier verrig vir meeste platformhoogtes, maar oortref die aanvanklike waaier se werksverrigting by die laagste platformhoogte en mees ongunstige toestande. Hierdie bewys die vermoëns van die vervangingswaaier as ‘n randwaaier.