Horst Richter

An analysis of the cold spray process and its coatings

In this study, computational fluid dynamics (CFD) and extensive spray tests were performed for detailed analyses of the cold spray process. The modeling of the gas and particle flow field for different nozzle geometries and process parameters in correlation with the results of the experiments reveal that adhesion only occurs when the powder particles exceed a critical impact velocity that is specific to the spray material. For spherical copper powder with low oxygen content, the critical velocity was determined to be about 570 m/s. With nitrogen as the process gas and particle grain sizes from 5–25 µm, deposition efficiencies of more than 70% were achieved. The cold sprayed coatings show negligible porosity and oxygen contents comparable to the initial powder feedstock. Therefore, properties such as the electrical conductivity at room temperature correspond to those of the bulk material. The methods presented here can also be applied to develop strategies for cold spraying of other materials such as zinc, stainless steel, or nickel-based super-alloys.

Flooding in tubes and annuli

Abstract The limitation of vertical countercurrent flow, called flooding, is important for the operation of Emergency Core Cooling Systems in Nuclear Reactors. A new flooding correlation is presented which solves the obvious contradiction between the Wallis correlation and the study by Pushkina and Sorokin concerning the scaling question at zero penetration of liquid. In addition, this flooding correlation is applicable for partial delivery in pipe and annuli experiments as long as the liquid penetrates in the form of a film along the walls.

SEPARATED TWO-PHASE FLOW MODEL: APPLICATION TO CRITICAL TWO-PHASE FLOW

Abstract A separated flow model has been developed to allow the calculation of critical flow rates for steam-water mixtures. This model considers hydrodynamic as well as thermal non-equilibrium effects which are present due to rapid depressurization. Thus, this model incorporates interphase interaction terms for momentum, energy and mass. The mass transfer, evaporation or condensation rate, is coupled with the heat transfer between the two phases. Certain empiricisms are necessary to be included into this model, e.g. the size and number of nucleation sites at the onset of flashing. Transitions from one flow regime to the other are assumed to occur at certain void fractions. Justification of these assumptions is only possible by comparison with experimental results of different authors which in general shows good agreement.

Prediction of bubble concentration profiles in vertical turbulent two-phase flow

Abstract In vertical bubbly flow, the bubbles are not distributed evenly across the flow section. Several investigators have observed a wall-skewed bubble concentration profile in a vertical upward flow. This paper presents an analysis that predicts this type of bubble distribution by incorporating into the equation of motion a lateral force due to the relative velocity of the two phases and the eddy diffusivity of the liquid. Comparison of analysis and experiment shows good agreement.

An advanced integrated biomass gasification and molten fuel cell power system

Abstract Biomass has recently received considerable attention as a potential substitute for fossil fuels in electric power production. Renewable biomass crops, industrial wood residues, and municipal wastes as fuels for production of electricity allow substantial reduction of environmental impact. High reactivity of biomass makes it relatively easy to convert solid feedstocks into gaseous fuel for subsequent use in a power cycle. So far most of the studies were focused on investigating performance and economics of biomass gasifiction–gas turbine systems. A general conclusion resulting from these studies is that the combination of biomass gasifiers, hot gas cleanup systems, and advanced gas turbines is promising for cost competitive electric power generation 1 , 2 . In this paper another concept of biomass fueled power systems is considered, namely biomass gasification with a molten carbonate fuel cell (MCFC). Comparison between two concepts is made in terms of efficiency, feasibility, and process requirements. As an example of such a system, a highly efficient novel power cycle consisting of the Battelle gasification process, a molten carbonate fuel cell, and a steam turbine is introduced. The calculated efficiency is around 53%, which exceeds efficiencies of traditional designs 1 , 3 considerably. Finally, an economic analysis and electricity cost projection are performed for a power plant consuming 2000 tons of biomass per day. Results are compared with those for more traditional integrated biomass gasification/gas turbine systems and for coal fueled cycles.

A pragmatic analysis and comparison of HVOF processes

A number of high-velocity oxygen-fuel flame (HVOF) systems have evolved during the last 9 years. The most advanced is now challenging the coating qualities produced by the very successful detonation (D-Gun) process. The fundamentals of these various processes are described and compared. A mathematical analysis of an established HVOF gun is profiled. Gas and particle temperatures, velocities, pressures, and Mach numbers are calculated and plotted at various points within the gun and spray stream. Significantly, all measured values were in close agreement with calculated and predicted values. Flow patterns and shock-wave phenomena are also described and compared with actual observations.

Countercurrent gas-liquid flow in parallel vertical tubes

Abstract The subject of this paper is the flow between an upper reservoir, containing a liquid, and a lower reservoir, containing a gas, interconnected by parallel vertical tubes. The characteristics of the combined system are predicted from a knowledge of the behavior of flow in individual tubes. Numerous modes of possible operation are described analytically and demonstrated experimentally. The effects of system geometry, changes in gas supply characteristics, operating procedure and two-phase flow regimes on the transitions between modes and system stability are presented. Predictions are made for the limiting case of a large number of identical parallel channels.

INSTABILITY ANALYSIS OF THE TRANSITION FROM BUBBLING TO JETTING IN A GAS INJECTED INTO A LIQUID

Abstract Submerged gas injection into liquids is a widely applied processing technique. At low gas flow rates, a bubble plume forms in the liquid. With increasing gas flow rates, the gas bubbles in the plume coalesce and a gas jet is formed. Transition from bubbling to jetting occurs in the transonic region. To date, there is no sufficient theoretical explanation for this transition. In this paper, a basic theory is developed to explain the transition from bubbling to jetting. The instability of a circular compressible gas jet in a liquid was studied. For the axisymmetric mode, it was found that there is a peak growth rate for both the temporal and spatial instabilities when the Mach number approaches unity. The instability quickly reduces to vanishing values at supersonic gas velocities. However, in the supersonic region, it was shown that the helical instability mode may become important. Gas pressure perturbations have a destabilizing effect in the subsonic region but a stabilizing effect in the supersonic region. The problem of absolute instability was studied in order to explain the physical phenomenon of the transition from bubbling to jetting. Absolute instability was found in the subsonic region and a gas jet always breaks up into bubbles in the subsonic region. No absolute instability was found in the supersonic region, and the gas jet may remain stable. This transition from absolute to non-absolute instability occurs in the transonic region which was observed to be the transition from bubbling to jetting.

High efficiency coal-fired power plant of the future

In this work a novel coal fueled power plant, which combines coal gasification and solid oxide fuel cell technology, is presented. The cycle is a modification of an earlier one proposed by the same authors [Lobachyov and Richter, Journal of Energy Resources Technology, 1996, 188]. It retains most of the elements of the previous plant: Conoco CO2 acceptor gasification process, solid oxide fuel cell and state-of-the-art gas turbines. A key difference between the previous and the new cycle is a different heat recovery concept. Instead of using the sensible heat of the cathode exhaust for the coal gasification process, the fuel not oxidized in the fuel cell is burnt to provide this heat. This new concept allows us to simplify the cycle and makes its performance less dependent on the pressure drop in different components of the cycle. Further, it considerably reduces the exergy loss in the gas turbine by eliminating exergetically inefficient turbine blade cooling. The new cycle achieves a gain in the overall efficiency of more than two percentage points under the same operating parameters. At a maximum system pressure of 10 bar and an anticipated solid oxide fuel cell voltage of 0.7 V the overall thermal efficiency is approximately 63%.

NUMERICAL MODELING OF HIGH-SPEED FLOWS OVER A MICROSPHERE IN THE SLIP AND EARLY TRANSITION FLOW REGIMES

The focus of this paper is on using computational fluid dynamics to investigate the drag and convection heat transfer of high-speed flows over a microsphere. The flow under investigation is steady-state, subsonic, transonic or supersonic laminar flow over a sphere. Due to the small size of the particle (< 80 microns), the flow is in the slip and early transition regimes. Typical Reynolds number based on sphere’s diameter is between 10 and 6000, and the Knudsen number is between 0.001 and 0.75.For the slip flow as well as the early transition regimes, instead of using the Direct Simulation Monte Carlo methods (DSMC) or lattice Boltzmann methods, we use ANSYS FLUENT, a Navier-Stokes-Fourier solver with the first-order velocity-slip and temperature-jump boundary conditions. In order to capture the non-equilibrium effects in the Knudsen layer, a constitutive scaling model for gas viscosity and conductivity is also implemented in the CFD model.CFD simulations were performed at the free-stream Mach number from 0.6 to 3.0, with particle diameter from 1 to 80 microns and the Knudsen number from 1.4 × 10−3 to 0.14. The CFD results are in good agreement with experimental data. The deviations from the data are within 10%.The numerical model also provides additional insight to the concept of the thermal recovery temperature in high-speed convection. Due to the nature of the temperature-jump boundary condition, the thermal recovery temperature in the slip flow regime can be obtained numerically only by solving the conjugate heat transfer problem. A “thin-wall” model is introduced in this paper in order to determine the thermal recovery temperature (or recovery factor) for the given Mach and Reynolds numbers.Although a number of publications have been devoted to particle drag correlations as functions of particle Reynolds and Mach numbers, the dependence of drag on particle temperature has not been investigated. By using the rarefied gas flow model in this study, we have not only confirmed that the drag increases as particle temperature goes higher, but also found that rate of drag increase is higher for the transonic than for the supersonic flows.Copyright