High-temperature Turbulent Flows Research Articles

Experimental method for investigating wear of porous thermal insulation systems exposed to realistic, hot, turbulent gas flow

A realistic wear test was developed for porous thermal insulation systems exposed to high temperature turbulent gas flow, because it is essential for the development of existing and new concepts of such insulation and therefore also for the performance of processes that depend on such insulation. Wear is crucial and often dominant for the long-term performance of thermal insulation and, because of the complex nature of insulation wear under exposure of high-temperature turbulent flow, realistic testing capability is a necessary tool for improvement. A test rig was developed to subject fibrous ceramic insulation, the most encountered type of thermal insulation, to conditions representative for in-service use and to enable investigation of the occurring phenomena and behaviour. This rig can accommodate a range of different insulation configurations and is compatible with many turbulent flow sources. This test rig, its components, the experimental procedure, its accuracy and representative results are presented.

Generation and Characterization of Chaotic Convection in Collisional Plasma

Turbulence in the plasma sheath around reentry vehicles is known to contribute to radio-communications blackout, but a practical laboratory model of that extreme environment remains elusive. Herein, we present a table-top plasma system with sustained, chaotic convection for that purpose. Strong sound waves exert acoustic radiation pressure on gradients within the plasma and are shown to drive sufficient convection to cause abrupt and chaotic variation in the plasma properties. The volume-averaged plasma conductivity and collision time are determined in real time by phase-sensitive detection of a microwave probe signal. The experiment provides unique opportunities to study transmission into plasma conditions that can inform detailed models of high-temperature turbulent flows.

Online-CPD-Coupled Large-Eddy Simulation of Pulverized-Coal Pyrolysis in a Hot Turbulent Nitrogen Jet

ABSTRACTThe pyrolysis characteristics of pulverized-coal particles in a hot turbulent nitrogen jet were investigated using large-eddy simulation (LES). In the present study an advanced pyrolysis model, the chemical percolation devolatilization (CPD) model, has been incorporated into LES in real time. The simulation results of the developed online-CPD-coupled LES were used to calibrate the kinetic parameters of the conventional single first-order reaction model (SFOM). Through the comparison between the CPD-coupled LES and the LES using the SFOM model, the CPD-coupled LES approach is found to be able to give a better prediction on particle pyrolysis in the high-temperature turbulent flow. Finally, the effects of important parameters, including the particle diameter, coal type, coal-feeding rate, carrier-phase velocity, and pyrolysis temperature, on the pulverized-coal pyrolysis process were investigated through parametric studies using the online-CPD-coupled LES method.

Effect of Variable Properties Within a Boundary Layer With Large Freestream-to-Wall Temperature Differences

Modern gas-turbine engines are characterized by high core-flow temperatures and significantly lower turbine-surface temperatures. This can lead to large property variations within the boundary layers on the turbine surfaces. However, cooling of turbines is generally studied near room temperature, where property variation within the boundary layer is negligible. The present study first employs computational fluid dynamics to validate two methods for quantifying the effect of variable properties in a boundary layer: the reference temperature method and the temperature ratio method. The computational results are then used to expand the generality of the temperature ratio method by proposing a slight modification. Next, these methods are used to quantify the effect of variable properties within a boundary layer on measurement techniques, which assume constant properties. Both low-temperature flows near ambient and high-temperature flows with a freestream temperature of 1600 K are considered under both laminar and turbulent conditions. The results show that variable properties have little effect on laminar flows at any temperature or turbulent flows at low temperatures such that constant property methods can be validly employed. However, variable properties are seen to have a profound effect on turbulent flows at high temperatures. For the high-temperature turbulent flow considered, the constant property methods are found to overpredict the convective heat transfer coefficient by up to 54.7% and underpredict the adiabatic wall temperature by up to 209 K. Utilizing the variable property techniques, a new method for measuring the adiabatic wall temperature and variable property heat-transfer coefficient is proposed for variable property flows.

Fluctuating temperature measurement by a fine-wire thermocouple probe: influences of physical properties and insulation coating on the frequency response

Theoretical analysis of the frequency response of a fine-wire thermocouple probe has been further developed to investigate influences of the difference in physical properties between the two wires composing a butt-welded fine-wire thermocouple. The solution obtained demonstrates complex behaviour of the frequency response of the copper–constantan (type-T) thermocouple whose physical properties differ greatly between the two constitutive wires. It was also shown that, in an air flow, the frequency response is almost uniform in the cross-section of the thermocouple wire, but, in a water flow, it becomes remarkably nonuniform. The present theoretical analysis provides useful information for improving the reliability of fluctuating temperature measurement in high-temperature turbulent flows.

A Parametric Study of Spray Drying of a Solution in a Pulsating High-Temperature Turbulent Flow

Spray drying of a concentrated common salt (NaCl) solution carried out in the intense oscillating high-temperature turbulent flow field generated in the tailpipe of a pulse combustor was simulated. Simulation of such transport process problems is especially crucial since the environmental conditions are too hostile for detailed and reliable measurements. The momentum, heat, and mass transfer processes between the gas and droplet phases during drying were simulated using a computational fluid dynamic solver. The simulated profiles of flow field, temperature, and humidity of gaseous phase, and particle trajectories in a drying chamber are presented and discussed. The effects of gas temperature, pulse frequency and amplitude, and gas mass flow rate on the transient flow patterns, droplet trajectories, and overall dryer performance were investigated. Different turbulence models were also tested. Simulation results show that the flow field and droplet drying conditions vary widely during a single pulsating period. Very short drying times and very high drying rate characterize pulse combustion spray drying. Thus, pulse combustion drying can be applied to drying of fine droplets of highly heat-sensitive materials although the jet temperature initially is extremely high.

Application of Acoustic Emission for Monitoring the HVOF Thermal Spraying Process

This research aims to characterise and quantify the acoustic emission (AE) generated during the high velocity oxy-fuel (HVOF) thermal spraying process, recorded using piezoelectric AE sensors. The HVOF process is very complex involving high temperature turbulent flow through a nozzle with entrained particles, the projection of these particles, and their interaction with the target surface. Process parameters such as gun speed, oxy-fuel pressure and powder specification affect various characteristics of the coating, including thermal residual stresses; the lamellar microstructure and the topology and geometry of pores, all formed when the fused powder hits the surface, forming “splats”. It is widely acknowledged in the thermal spray industry that existing quality control techniques and testing techniques need to be improved. New techniques which help to understand the effects of coating process parameters on the characteristics of the coating are therefore of value, and it was anticipated that recording the AE produced when the fused particles contact the surface would aid this understanding. As a first stage, we demonstrated here that AE associated with particle impact can, in fact, be discerned in the face of the considerable airborne and structure-borne noise. In order to do this, a new test method using a masking sheet with slits of varying size was developed. Thermal spraying was carried out for a range of spray gun speeds and process parameters. The AE was measured using a broad band AE sensor positioned on the back of the sample as the spot was traversed across it. The results show that the amplitude and energy of the AE signals is related to the spray gun speed, powder used and the oxy-fuel pressure. Using a simple geometrical model for particle impact, the measured AE was found to vary with the energy and number of particles impacting on the sample in a predictable way.

Influences of Physical Properties and Coating on Frequency Response of Fine-Wire Thermocouple Probes

Theoretical analysis of the frequency response of a fine-wire thermocouple probe was performed. A strict solution of the response was derived and used to investigate influences of the difference in physical properties between two butt-welded thermocouple wires. The solution obtained enables us to understand complex behaviors of the frequency response, even if the physical properties of the two thermocouple wires are greatly different as seen in the copper-constantan (type-T) thermocouple. Additionally, the frequency response at an arbitrary point in the cross-sectional area of the thermocouple wire was investigated. The result shows that a common assumption that the cross-sectional temperature distribution is kept uniform becomes invalid in a high-frequency region, though the nonuniformity can usually have negligible effects on the thermocouple response. The present theoretical analysis provides useful information for revealing the response characteristics of a fine-wire thermocouple probe and for improving the reliability of the fluctuating temperature measurement in high-temperature turbulent flows.

Simultaneous measurement of velocity and temperature in high-temperature turbulent flows: a combination of LDV and a three-wire temperature probe

This paper deals with a simple and reliable technique for simultaneous measurement of velocity and temperature in high-temperature turbulent flows, including combustion. The technique is based on the combination of laser Doppler velocimetry and a digitally compensated fine-wire thermocouple. For temperature measurement, a two-thermocouple probe with a fine cold wire [Tagawa et al. (1998) Rev Sci Instrum 69: 3370–3378] is used, which enables in situ measurement of thermocouple time constants and accurate compensation of the thermocouple response. When tested in a turbulent wake behind a heated cylinder, the technique proves to be highly reliable and effective for investigating heat transport processes in various non-isothermal turbulent flows.

Simultaneous Measurement of Velocity and Temperature in High-Temperature Turbulent Flows.

This paper deals with a simple and reliable technique for simultaneous measurement of velocity and temperature in high-temperature turbulent flows, including combustion. The technique is based on the combination of laser Doppler velocimetry (LDV) and a digitally compensated fine-wire thermocouple. A central issue of this study is to find the applicability and reliability of the technique proposed. For this purpose, a two-thermocouple probe with a fine cold wire (Tagawa, Shimoji and Ohta, 1998), which enables in situ measurement of thermocouple time constants and accurate compensation of the thermocouple response, is combined with LDV. The technique is tested in a turbulent wake behind a heated cylinder, whose thermal field is of ordinary temperature and fluctuates with relatively small amplitude. This enables critical assessment of the measurement accuracy. The results show that the technique is highly reliable and effective for investigating heat transport processes in various non-isothermal turbulent flows.

Numerical simulation of the formation of nitric oxides in the combustion of coal-dust fuel

A mathematical model and a method of calculating two-phase flows and heat and mass transfer in industrial combustion chambers operating on coal-dust fuels are given. The numerical model of coal-dust combustion in a high-temperature turbulent flow is based on three-dimensional steady-state equations of mechanics of heterogeneous media and describes the processes of yield and combustion of fuel volatiles, afterburning of the coke residue, radiative heat transfer in the combustion chamber, and the effect of the disperse phase on the turbulent structure of the carrier medium. To predict the concentration of nitric oxides inside a combustion chamber, an effective numerical model of NO formation during jet burning of coal dust, which is based on the Mitchell-Tarbell kinetic scheme, is proposed. The results obtained in the use of the proposed complex numerical model are in good agreement with experimental data for industrial combustion chambers. The degree of detail ensured by the calculations allows one to make effective decisions to organize coal-dust burning with a decreased yield of hazardous nitric oxides.

Two-thermocouple probe for fluctuating temperature measurement in combustion—Rational estimation of mean and fluctuating time constants

A theoretical basis for the application of a two-thermocouple probe, comprised of two fine-wire thermocouples of different diameters, to carry out fluctuating temperature measurements is established with the aid of the method of least squares. Based on this theory, the simultaneous in situ reconstruction of thermocouple time constants and compensated temperatures has been realized without using any geometrical features of the two thermocouples such as wire diameters. Thus, there is no need to introduce a so-called time-constant ratio into the reconstruction scheme. Consequently, the essential drawbacks in the two-thermocouple techniques proposed so far can be overcome. In addition, a simple and reliable method of correcting for heat loss due to thermal radiation from the thermocouples is devised, which can be incorporated into the original reconstruction scheme with little modification. The scheme developed has been tested in a fluctuating temperature field with a diffusion flame formed in a two-dimensional wind tunnel. It is shown that the present scheme is successful and even allows study of the characteristics of time-constant fluctuations in high-temperature turbulent flows.

Two-Wire Thermocouple for Fluctuating Temperature Measurements in Combustion. Rational Reconstruction of Mean and Fluctuating Time Constants.

A theoretical basis for the application of a comprised of two fine thermocouples of different diameters to fluctuating temperature measurements is established with the aid of the least squares method. Based on this theory, the simultaneous in situ reconstruction of thermocouple time constants and compensated temperatures is realized without using any geometrical features of the two thermocouples such as wire diameters. Thus, introduction of a 'time-constant ratio' into the reconstruction scheme is unnecessary. Consequently, problems in two-wire thermocouple techniques proposed so far can be overcome. In addition, a simple and reliable method for correcting heat loss due to thermal radiation from the thermocouples is devised, which can be incorporated into the reconstruction scheme with little modification. The developed scheme is tested in a fluctuating temperature field with a diffusion flame formed in a two-dimensional wind tunnel, It is shown that the present technique is successful and allows us to investigate the characteristics of time-constant fluctuations in high-temperature turbulent flows.

Experimental study of the effect of acoustic oscillations on heat transfer in a gas flow

The influence of acoustic oscillations on heat transfer in a high-temperature turbulent flow is determined. It is shown how the heat transfer at resonance frequencies differs at nodes and antinodes of the standing acoustic wave.