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  • About
  • The Global ETD Search service is a free service for researchers to find electronic theses and dissertations. This service is provided by the Networked Digital Library of Theses and Dissertations.
    Our metadata is collected from universities around the world. If you manage a university/consortium/country archive and want to be added, details can be found on the NDLTD website.
1

Heat transfer in inundation and drainage flows associated with power condensers

Howell, Christopher John January 1992 (has links)
No description available.
2

Thermal-hydraulic designs of finned heat-exchangers experiencing free or forced convection

Naik, S. January 1987 (has links)
No description available.
3

Two-phase local heat transfer correlations for non-ozone depleting refrigerant-oil mixtures

Boissieux, Xavier January 1998 (has links)
This thesis describes the work undertaken over a 3 year period under a Department of the Environment, Transport and the Regions (DETR) Partners in Technology programme in collaboration with four industrial partners. The aim of the work was to investigate the two-phase flow heat transfer and fluid dynamic performance of the non-ozone depleting refrigerants and lubricating oil mixtures in a horizontal tube. A single tube test facility was designed and built to determine local heat transfer coefficients and overall pressure drop for evaporation and condensation. A circulating pump system pressurised through an accumulator charged with nitrogen was selected to enable accurate setting of the evaporation and condensation conditions. The system was validated using R22. Tests were undertaken for R404A, R407C, Isceon 59 with a Polyol-ester lubricating oil. The experimental data consist of two-phase local heat transfer coefficients and overall pressure drops inside a smooth copper tube. Numerous data were obtained for evaporation and condensation, and constituted a large database of two-phase heat transfer coefficients. The experimental results were compared with several existing correlations traditionally used in heat exchanger design, to assess their suitability with the new refrigerants. As a result of these comparisons offering a poor agreement, a model characterising the variation of the local heat transfer coefficient was developed. The evaporation model consists in a modification of the Kattan et al. model (1998a, b & c). The new model predicts the experimental results well, with a standard deviation of 6.1% for refrigerant R407C and Isceon 59. The effects of lubricating oils on two-phase flow heat transfer are also discussed qualitatively. This work provides a design tool when dealing with the non-ozone depleting refrigerants, and recommendations are made on the use of several correlations.
4

Multi-year Operation Effect of Geothermal Heat Exchanger on Soil Temperature for Unt Zero Energy Lab

Walikar, Vinayak P. 12 1900 (has links)
Ground source heat pump (GSHP) uses earth’s heat to heat or cool space. Absorbing heat from earth or rejecting heat to the earth, changes soil’s constant temperature over the multiple years. In this report we have studied about Soil temperature change over multiple years due to Ground loop heat exchanger (GLHE) for Zero Energy Research Laboratory (ZØE) which is located in Discovery Park, University of North Texas, Denton, TX. We did 2D thermal analysis GLHP at particular Depth. For simulation we have used ANSYS workbench for pre-processing and FLUENT ANYS as solver. TAC Vista is software that monitors and controls various systems in ZØE. It also monitors temperature of water inlet/outlet of GLHE. For Monitoring Ground temperatures at various depths we have thermocouples installed till 8ft from earth surface, these temperatures are measured using LabVIEW. From TAC Vista and LabVIEW Reading’s we have studied five parameters in this report using FLUENT ANSYS, they are; (1) Effect of Time on soil Temperature change over Multi-years, (2) Effect of Load on soil temperature change over Multi-years, (3) Effect of Depth on soil temperature change over Multi-years, (4) Effect of Doubling ΔT of inlet and outlet of GLHE on soil temperature change over multi-years and (5) Effect on soil temperature change for same ZØE Laboratory, if it’s in Miami, Florida. For studying effect of time on soil temperature change for multi-years, we have varied heating and cooling seasons. We have four cases they are Case A: GSHP always “ON” (1) 7 months cooling and 5 month cooling and (2) 257 days are cooling and 108 days heating. Case B: GSHP “OFF” for 2 months (1) 7 months cooling and 3 months heating and (2) 6 months cooling and 4 month heating. For Studying Effect of Load on soil temperature change over multi-years, we have considered maximum temperature difference between inlet and outlet for heating and cooling season for simulation. For studying effect of doubling ΔT of inlet and outlet of GLHE, we have doubled the temperature difference between inlet and outlet of GLHP. There will be soil temperature change over year at various depths. For studying Effect of Depth on soil temperature change for multi-years, we have consider 5 depths, they are 4ft, 6ft, 8ft, 110ft and 220ft. The Densities of soil are known from site survey report of ZØE GSHP manufacturers till depth of 13ft. For studying effect of soil temperature over multi-years for same ZØE in Miami, Florida, we have considered equivalent cooling and heating season from weather data for past one year and assuming same number of days of cooling and heating for next 20 years we have simulated for soil temperature change.
5

Effects of particle concentration and surfactant use in convective heat transfer of CuO nanofluids in microchannel flow

Byrne, Matthew Davidson 17 June 2011 (has links)
Heat exchange systems used in everything from cars to microelectronics have rapidly advanced in recent years to offer high heat transfer rates in increasingly smaller sizes. However, these systems have become essentially optimized using conventional heat transfer fluids. To test the viability of nanofluids as a new heat transfer fluid, an experimental investigation was designed using a constant pressure drop configuration to drive flow into a heated square microchannel test section. The experimental trials included seven different test fluids tested over varying concentrations and surfactant use. Two identical test sections were used to collect results on heat transfer rates, pressure drop, mass flowrate and pumping power for all fluids. These results show a heat transfer improvement for nanofluids of 8-16% over pure water, with no meaningful increase in pumping power. This result is highly desirable, as it indicates an easily obtainable heat transfer improvement without an associated pumping cost increase. Importantly, the experiment shows the potential viability of nanofluids for heat transfer applications, while acknowledging limitations such as long term nanofluid stability. / text
6

The Radiative and Conductive Heat Exchange of a Cold Desert

Martinez, Luis R. 01 May 1975 (has links)
The heat exchange during dry conditions in Curlew Valley for selected clear days is determined from recordings in 10 minute intervals . The two heat budget components , radiative exchange and heat flow in the soil, are studied in detail. Maximum surface temperature due to radiative exchange above, assuming no heat loss by other components, is determined. Reduction of surface temperature by heat conduction into the ground was calculated as a second step to solve the heat budget equation. Substantial heat flow throughout a day is restricted to the upper 10 em of the soil. Diffusivity of the soil was determined from soil temperature using phase and amplitude equations, of which the first gave better results.
7

Evapotranspiration Estimates from the Water Balance and Equilibrium Models

Wilson, Richard Garth 05 1900 (has links)
<p> This thesis examines the field performance of the water balance and equilibrium evapotranspiration models, and defines the environmental conditions for which they provided accurate estimates of water loss from a corn crop in Southern Ontario.</p> <p> It is shown that the water balance model should be used only when surface runoff is measured and drainage is negligible. An error analysis indicated that soil moisture change could be estimated within 10 percent when measurements were conducted at six sites every eight days.</p> <p> The equilibrium model predicted daily evapotranspiration within 6 percent when the latent heat exchange utilized between 65 and 80 percent of the available energy, indicating that the model can be applied within temperature limits of 17° and 32°C.</p> / Thesis / Doctor of Philosophy (PhD)
8

Numerical Modelling of Multiple Inclined Borehole Heat Exchangers / Numerical Modelling of Multiple Angled Borehole Heat Exchangers

Deacon, Daniel January 2023 (has links)
This research describes the development and application of a numerical modelling method for angled borehole heat exchangers in ground-source heat pump systems. Inclining the boreholes relative to the vertical axis presents an opportunity to reduce the ground level footprint of the borehole field thus allowing for the installation of geothermal systems in retrofit applications or under buildings with small footprints. The commercial code COMSOL was used to develop the computational model. A series of validation and verification studies were performed to ensure the accuracy of the modelling approach. Simulations were conducted under constant and transient heat injection, where the effect of energy load imbalance is analyzed. Additionally, the effect of discontinuous loading with natural and forced recovery cycles is investigated. When exposed to a constant heat injection rate, configurations of angled borehole heat exchangers initially perform the same as vertical borehole heat exchangers. Then, there is a relatively short period where the angled configurations have slightly decreased performance due to increased thermal interaction in the near surface. At longer times, however, there is a significant benefit in using angled borehole heat exchangers as a result of the increased ground volume in the lower portion of the field. Under transient loading conditions, the conclusions were the same as constant heat injection, although the differences were smaller when the energy loading was balanced. However, when the loading was cooling dominated, by year 10 there was a significantly better performance observed for the angled boreholes. This indicates that the configurations of angled borehole heat exchangers can withstand a higher intensity of imbalanced energy loads compared to vertical configurations. Discontinuous loading was investigated by varying the length of time heat injection would occur on a daily basis. These daily perturbations led to small performance losses in the angled boreholes due to the increased thermal interaction in the near surface. Furthermore, imposing a forced recovery on the system by circulating fluid while heat injection was off did not significantly affect the fluid temperature or ground temperature. / Thesis / Master of Applied Science (MASc)
9

Värmeväxling med torkluft från Valmets pilotmaskin TM1 : Återvinning av spillvärme vid torkning av mjukpapper / Heat exchange using exhaust air from Valmet's pilot tissue machine TM1 : Recovering waste heat from tissue drying process

Iverlund, Per January 2014 (has links)
Idag ställs stora krav på företag att vara miljömedvetna och resurseffektiva i sin verksamhet. Vid mjukpapperstillverkning används stora energimängder och inom industrin har det under många år funnits en strävan att energieffektivisera processer och att tillvarata spillenergi, bland annat genom värmeåtervinning. Valmet Tissue Technology Center i Karlstad har idag en pilotmaskinen utan värmeåtervinningssystem. Den här undersökningen gjordes i syfte att ta reda på hur stor återvinningspotentialen är för värmeväxling i pilotmaskinens torkpartier, bestående av Yankeekåpan och TAD-cylindrarna. Tre maskinkoncept undersöktes: DCT®, NTT™ och TAD. Entalpi och effekt i utgående torkluft bestämdes utifrån dess massflöde, temperatur och fukthalt för respektive koncept och torkparti. För att beräkna återvinningspotential i luftflödena undersöktes effektbehov för avsättningsalternativ bestående av förvärmning av förbränningsluft och make-up air, uppvärmning av radiatorvatten samt ånggenerering med Waste Heat Steam Generator. Effekt- och energimässig återvinningspotential beräknades dels teoretiskt och dels reellt med simuleringsprogram för befintliga produkter. Ekonomisk besparing från minskad energianvändning samt investeringskostnad för respektive avsättningsalternativ och torkparti beräknades. Paybacktid användes som mått på ekonomisk lönsamhet. Utöver ovanstående undersöktes om det finns ett samband mellan aktivitet på pilotmaskinen och effekttopparna för fjärrvärmeanvändning i den aktuella byggnaden. Störst entalpi har utgående torkluftsflöden från Yankeekåpan vid DCT-körning följt av NTT-körning. Den största effektmässiga återvinningspotentialen för luftförvärmning finns i TAD-cylinder 1 men betydande tryckfall uppstår i värmeväxlaren på grund av stora luftflöden. Waste Heat Steam Generator kan inte användas ihop med pilotmaskinen på grund av för låg entalpi och effekt i utgående torkluft. Generellt är den reella återvinningspotentialen vid luftförvärmning mindre än den teoretiskt beräknade. Totalt innebär värmeväxling från Yankeekåpan den största årliga energibesparingen vid luftförvärmning. Reell energibesparing vid luftförvärmning i Yankeekåpan är 55 MWh per år vilket motsvarar 4 300 kg gasol. För uppvärmning av radiatorvatten finns den största effekt- och energimässiga återvinningspotentialen vid värmeväxling i Yankeekåpan under DCT-körning följt av NTT-körning. TAD-körning innebär en mindre återvinningspotential vid värmeväxling både i Yankeekåpan och TAD-cylindrarna. Totalt innebär värmeväxling från Yankeekåpan den största årliga energibesparingen vid uppvärmning av radiatorvatten. Reell energibesparing vid uppvärmning av radiatorvatten med torkluft från Yankeekåpan är 153 MWh fjärrvärme per år. Årlig kostnadsbesparing vid luftförvärmning är 43 000 SEK och vid uppvärmning av radiatorvatten 63 400 SEK. Paybacktid för investering i luftförvärmning är 2,6 år och för uppvärmning av radiatorvatten 4,4 år. Ett samband mellan att pilotmaskinen körs och hög fjärrvärmeförbrukning finns, dock är det rutinförändringar kring lokalventilering under körning som främst kan bidra till att sänka de högsta effekttopparna. / There are great demands on industrial companies today to be environmentally responsible and resource-efficient. Within the paper and tissue industry a lot of energy is being used in the processes and since many years there’s been a development towards a more efficient energy use, for example by recovery of waste heat. Valmet Tissue Technology Center in Karlstad, Sweden, has a pilot tissue machine without a waste heat recovery system. This thesis was carried out to investigate the waste heat recovery potential of the pilot machine’s drying sections, being the Yankee hood and TAD cylinders. Three different concepts of the machine were studied: DCT®, NTT™ and TAD. The enthalpy and heat flows of the exhaust air from the drying sections were calculated by the air’s mass flow, temperature and humidity. To calculate the heat recovery potential the heat demand for pre-heating combustion and make-up air, heating radiator water and generating steam by using a Waste Heat Steam Generator was investigated. The heat recovery potential was calculated theoretically but also simulated using programs for real heat exchangers. Economic savings from reduced energy use and investment costs was used to calculate the payback time for each investment alternative. As a separate task the maximum district heating loads in the facility of the pilot machine was cross checked with the pilot machine activity, to clarify any relationship between them. The largest enthalpy was found to be in the exhaust air from the Yankee hood when running the DCT concept followed by the NTT concept. However, the largest heat flow is in the exhaust air from the first TAD cylinder. The air from the first TAD cylinder meant the largest heat recovery potential but caused big pressure drops in the heat exchanger. The Waste Heat Steam Generator cannot be used together with the pilot machine due to too low enthalpy and heat flow in the exhaust air from the drying sections. In general the real heat recovery potential when pre-heating combustion and make-up air is smaller than the theoretical potential. In total, heat exchange using exhaust air from the Yankee hood means the largest energy saving when pre-heating air. The real energy saving when pre-heating air using outgoing air from the Yankee hood is 55 MWh per year, meaning 4 300 kg of propane. The largest potential for heating radiator water occurs when using exhaust air from the Yankee hood when running the DCT concept followed by the NTT concept. The TAD concept means smaller heat recovery potential regardless of air from the Yankee hood or TAD cylinders is being used. In total, heat exchange using outgoing air from the Yankee hood means the largest yearly energy saving when heating radiator water, the real energy saving in district heating being 153 MWh per year. The economic saving when pre-heating air is 43 000 SEK per year and when heating radiator water 63 400 SEK per year. The payback time when investing in pre-heating air is 2,6 years and when heating radiator water 4,4 years. A relationship between running the pilot machine and big loads of district heating use can be seen. However, changing the routines of ventilation in the machine hall during trial days would probably be the easiest way to reduce the maximum loads.
10

Boreal land surface water and heat balance : Modelling soil-snow-vegetation-atmosphere behaviour

Gustafsson, David January 2002 (has links)
The water and heat exchange in thesoil-snow-vegetation-atmosphere system was studied in order toimprove the quantitative knowledge of land surface processes.In this study, numerical simulation models and availabledatasets representing arable land, sub-alpine snowpack, andboreal forest were evaluated at both diurnal and seasonaltimescales. Surface heat fluxes, snow depth, soil temperatures andmeteorological conditions were measured at an agriculturalfield in central Sweden during three winters and two summersfrom 1997 to 2000 within the WINTEX project. A one-dimensionalsimulation model (COUP) was used to simulate the water and heatbalance of the field. Comparison of simulated and measured heatfluxes in winter showed that parameter values governing theupper boundary condition were more important for explainingmeasured fluxes than the formulation of the internal mass andheat balance of the snow cover. The assumption of steady stateheat exchange between the surface and the reference height wasinadequate during stable atmospheric conditions. Independentestimates of the soil heat and water balance together with thecomparison of simulated and measured surface heat fluxes showedthat the eddy-correlation estimates of latent heat fluxes fromthe arable field were on average 40 % too low. The ability of a multi-layered snowpack model (SNTHERM) tosimulate the layered nature of a sub-alpine snowpack wasevaluated based on a dataset from Switzerland. The modelsimulated the seasonal development of snow depth and densitywith high accuracy. However, the models ability to reproducethe strong observed snowpack layering was limited by theneglection of the effect of snow microstructure on snowsettling, and a poor representation of water redistributionwithin the snowpack. The representation of boreal forest in the land surfacescheme used within a weather forecast (ECMWF) model was testedwith a three-year dataset from the NOPEX forest site in centralSweden. The new formulation with separate energy balances forvegetation and the soil/snow beneath the tree cover improvedthe simulation of seasonal and diurnal variations in latent andsensible heat flux. Further improvements of simulated latentheat fluxes were obtained when seasonal variation in vegetationproperties was introduced. Application of the COUP model withthe same dataset showed that simulation of evaporation fromintercepted snow contributed to a better agreement with themeasured sensible heat flux above forests, but also indicatedthat the measurements might have underestimated latent heatflux. The winter sensible heat flux above the forest wasfurther improved if an upper limit of the aerodynamicresistance of 500 s m-1 was applied for stable conditions. A comparison of the water and heat balance of arable landand forest confirmed the general knowledge of the differencesbetween these two surface types. The forest contributed withconsiderably more sensible heat flux to the atmosphere than thearable land in spring and summer due to the lower albedo andrelatively less latent heat flux. Latent heat flux from theforest was higher in winter due to the evaporation ofintercepted snow and rain. The net radiation absorbed by theforest was 60 % higher than that absorbed by the arable land,due to the lower surface albedo in winter. Key words:soil; snow; land surface heat exchange;forest; arable land; eddy-correlation. / QC 20100614

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