The effect of surface renewal due to largescale eddies on jet impingement heat transfer

Rock failure mechanisms of flame-jet thermal spallation drilling—theory and experimental testing

The purpose of this research was to study the various factors affecting the economic and technical feasibility of Engineered Geothermal Systems, with a special emphasis on advanced drilling technologies The first part of the thesis was devoted to modeling and analysis of the technologies used to develop EGS projects Since the cost of completing wells is a major factor in determining the economic feasibility of EGS projects, it is vital to be able to accurately predict their costs Historic well cost data was analyzed to identify trends, and a drilling cost index for updating historic geothermal well costs to present day costs was developed The effects of different advanced drilling technologies on drilling costs were estimated and incorporated into a techno-economic model to estimate their impact, as well as the impact of advanced reservoir stimulation technologies, on EGS levelized electricity costs A technical analysis of geothermal binary Rankine cycle surface power plants was also performed to determine the effect of novel working fluids on plant efficiency for both suband supercritical binary cycles The objective of the second part of the thesis was the application of thermal spallation drilling to deep boreholes Thermal spallation is the fragmentation of a brittle solid into small, disc-like flakes by rapidly heating a confined fraction of the rock It was proposed that the necessary temperatures and heat fluxes needed to induce thermal spallation in the high pressure, high density deep borehole environment could be achieved using hydrothermal flame technologies An autoclave reaction system was designed and constructed to create flame jets in water at a pressure of 250 bar The temperatures of these flames were measured, and attempts were made to use the flames to spall small rock samples The experimental system was modified to study the centerline temperature decay of supercritical water jets injected at temperatures up to 525 C into ambient temperature water A device for measuring the heat flux from these jets was designed, constructed, and used to determine the heat transfer coefficients of the jets impinging against a flat surface Together, these studies indicate that the necessary temperatures and heat fluxes required to induce thermal spallation in rocks can be achieved in a deep borehole Thesis Supervisor: Jefferson W Tester Herman P Meissner Professor of Chemical Engineering

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Hydrothermal Spallation Drilling and Advanced Energy Conversion Technologies for Engineered Geothermal Systems

Jet mixing in a slot

We have examined the linear stability of the fully developed mixed-convection flow in a differentially heated tall vertical channel under non-Boussinesq conditions. The Three-dimensional analysis of the stability problem was reduced to an equivalent two-dimensional one by the use of Squire's transformation. The resulting eigenvalue problem was solved using an integral Chebyshev pseudo-spectral method. Although Squire's theorem cannot be proved analytically, two-dimensional disturbances are found to be the most unstable in all cases. The influence of the non-Boussinesq effects on the stability was studied. We have investigated the dependence of the critical Grashof and Reynolds numbers on the temperature difference. The results show that four different modes of instability are possible, two of which are new and due entirely to non-Boussinesq effects.

Stability of mixed-convection flow in a tall vertical channel under non-Boussinesq conditions

A generalized model for axisymmetric free jets which allows for change in fluid density with temperature is proposed by experiment. Experimental work was carried out to obtain data on jet velocity and temperature variations over a wide range of jet-to-ambient fluid density ratio (i.e. the initial density ratio) by use of nonisothermal free jets of burned gas exhausting into quiescent air as well as isothermal free jets of CO2-air mixture into air. Core lengths of velocity and temperature can be correlated as a function of initial density ratio and jet Reynolds number. The radial profiles of velocity and temperature in the fully-developed region can be expressed by the isothermal formulation using the corresponding jet half-radii as the characteristic radial distance. The jet half-radii and centerline decays can be described by new characteristic streamwise coordinates which take into account the effects of core length and initial density ratio. If the initial condition at the nozzle exit is specified, the present model can determine the radial distributions of velocity and temperature at any axial position.

A generalized model of the development of nonisothermal, axisymmetric free jets

Convective heat transfer is experimentally studied in the impingement region where a free jet of high-temperature burned gas impinges normally on a circular flat plate cooled from the back. Taking into account the effect of jet-to-ambient fluid density ratio on the jet development, a generalized, design-oriented model is proposed, which permits local prediction of the wall heat-fluxes over a wide range of the initial density ratio.

Characteristics of convective heat transfer in nonisothermal, variable-density impinging jets

A generalized eddy transfer coefficient model has been experimentally obtained for axisymmetric, nonisothermal variable-density jets, the initial density ratio of which ranges from 0.66 to 5.1. The local variation in the dynamic eddy transfer coefficients for momentum and heat has been measured in the developed region: both transfer coefficients on the jet axis are proportional to the square root of the initial density ratio, but are almost independent of the axial distance from the nozzle exit; the transfer coefficients for momentum and a chemical species remain constant in the inner region whereas the transfer coefficient for heat decreases slightly with increasing radial distance; all the transfer coefficients decrease rapidly in the outer region owing to the intermittent entrainment of the quiescent ambient fluid. The present model can perdict local values of the transfer coefficients over the range of initial density ratio from 0.66 to 5.1 if the initial jet condition alone is specified at a nozzle exit.

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Hiroyuki Shundoh

Papers

A generalized model of the development of nonisothermal, axisymmetric free jets

Characteristics of convective heat transfer in nonisothermal, variable-density impinging jets

An eddy transfer coefficient model for turbulent free jets with variable density