R. P. Roy

Turbulent Subcooled Boiling Flow—Experiments and Simulations

Experiments and simulations were carried out in this investigation of turbulent subcooled boiling flow of Refrigerant-113 through a vertical annular channel whose inner wall only was heated. The measurements used, simultaneously, a two-component laser Doppler velocimeter for the liquid velocity field and a fast-response cold-wire for the temperature field, and a dual-sensor fiberoptic probe for the vapor fraction and vapor axial velocity. In the numerical simulation, the two-fluid model equations were solved by the solver ASTRID developed at Electricite de France. Wall laws for the liquid phase time-average axial velocity and temperature were developed from the experimental data, and the turbulent Prandtl number in the liquid was determined from the wall laws. The wall laws and turbulent Prandtl number were used in the simulations. The wall heat transfer model utilized the measured turbulent heat flux distribution in the liquid. Results from the simulations were compared with the measurements

Isothermal and heated turbulent upflow in a vertical annular channel – Part I. Experimental measurements

Abstract The velocity and thermal fields were measured in isothermal and heated turbulent upflow of liquid Refrigerant-113 through a vertical annular channel of inner to outer radius ratio 0.415. A two-component laser Doppler velocimeter was used for the velocity measurements and, simultaneously, a cold-wire for the temperature measurements. The dimensions of the LDV measuring volume and the cold-wire, and their proximity to each other were important considerations. Also crucial to the measurements were the LDV coincidence time window and the temporal response of the cold-wire. Time-mean axial and radial velocities, axial and radial turbulent intensities, the single-point cross-correlation between axial and radial velocity fluctuations (∼axial Reynolds shear stress), and single-point cross-correlations between axial velocity and temperature fluctuations (∼axial turbulent heat flux) and radial velocity and temperature fluctuations (∼radial turbulent heat flux) were measured. Results are reported for Reynolds numbers at channel inlet of 22,800, 31,500, and 46,400 at annulus inner wall heat fluxes of 0, 9000 and 16,000 W m −2 . The measured radial turbulent heat flux distribution is compared with that calculated from an approximate form of the thermal energy balance equation in which the measured mean velocity and temperature values are used. Also reported is the radial distribution of turbulent Prandtl number estimated from the measurements.

Velocity Field in Turbulent Subcooled Boiling Flow

The velocity field was measured in turbulent subcooled boiling flow of Refrigerant-113 through a vertical annular channel whose inner wall was heated. A two-component laser Doppler velocimeter was used. Measurements are reported for two fluid mass velocities and four wall heat fluxes in the boiling layer adjacent to the inner wall as well as in the outer all-liquid layer. The turbulence was found to be inhomogeneous and anisotropic and the turbulent kinetic energy significantly higher than in single-phase liquid flow at the same mass velocity. The axial Reynolds shear stress in the liquid phase was observed to increase sharply near the inner wall. The near-wall velocity field appeared to be quite different from that in single-phase flow. It remains a challenge to separate the contributions of wall turbulence and bubble-induced pseudo-turbulence from the measurements which possibly represent a complicated interaction between the two.

Experiments on subcooled flow boiling heat transfer in a vertical annular channel

Abstract Experiments on subcooled flow boiling heat transfer are carried out in a vertical annular channel the inner wall of which is heated and the outer wall insulated. Refrigerant-113 is the working fluid. Flow boiling heat transfer data are reported at three mass velocities (579, 801, and 1102 kg m −2 s −1 ), three pressures (312, 277, and 243 kPa), and three inlet subcoolings (20.0, 30.0, and 36.5°C). A multiple-hysteresis phenomenon is identified. The measured wall heat transfer coefficients are compared with predictions by various correlations and an improvement of the Shah correlation for annuli is suggested.

A fast‐response microthermocouple

A fast‐response chromel‐alumel (and chromel‐constantan) microthermocouple is described and its dynamic characteristics are measured. The microthermocouple features a microdisk junction 0.08 mm in diameter and 2.5 μm thick. Its time constant in turbulent flow of liquid Refrigerant‐113 (a poor heat conductor) is measured to be ∼4.6 ms. This time constant was reduced to ∼3.4 ms with a phase‐lead compensation circuit. It was possible to distinguish between vapor and liquid phase temperatures in turbulent subcooled boiling flow of Refrigerant‐113 with the compensated microthermocouple.

The Flow Field and Main Gas Ingestion in a Rotor-Stator Cavity

The ingestion of mainstream gas into turbine rotor-stator disk cavities and simultaneously, the egress of cavity gas into the main gas path are consequences of the prevailing unsteady, three-dimensional flow field. To understand these processes, we are carrying out a study that combines experiments in a model single-stage axial turbine with computational fluid dynamic (CFD) simulations. The turbine stage features vanes, blades, and axially overlapping radial clearance rim seal. In this paper, we present time-resolved velocity maps, obtained by particle image velocimetry, of the flow in the disk cavity at four experimental conditions as defined by the main air flow rate, rotor speed, and purge air flow rate. Time-averaged but spatially local measurement of main air ingestion is also presented. Significant ingestion occurred at two of the four experimental conditions where the purge air flow rate was low — it is found that high tangential (swirl) velocity fluid intersperses with lower tangential velocity fluid in the rim region of the cavity. It is argued that the high tangential velocity fluid is comprised of the ingested main air, while the lower tangential velocity fluid is the indigenous cavity air. This interpretation is corroborated by the results of the unsteady, three-dimensional CFD simulation. When the purge flow rate was high, no ingestion occurred as expected; also, large-scale structures that were unsteady appeared in the cavity flow giving rise to large velocity fluctuations. It is necessary to obtain time-resolved information from experiments and computation in such a flow because even when the vane-blade relative position is matched during a particular experiment, the instantaneous flow field does not necessarily remain the same. As such, some of the flow patterns will be smeared out if the interrogation time scale is large.Copyright

Turbulent velocity field in isothermal and heated liquid flow through a vertical annular channel

Abstract Time-mean axial and radial velocities, axial and radial turbulent intensities and the single-point cross-correlation between axial and radial turbulent velocity fluctuations (∼axial Reynolds shear stress) were measured in unheated and heated turbulent upflow of liquid Refrigerant-113 through a vertical concentric annular channel. Time-mean temperature and temperature fluctuation intensity were measured in the heated flows. A two-component laser Doppler velocimeter was used for the velocity measurements and microthermocouples for temperature measurement. Results are reported for Reynolds numbers of 18 000, 28 450 and 40 480 at inner wall heat fluxes of 0, 9000 and 16 000 W m−2. Buoyancy effects were found on the time-mean velocity and turbulence fields, even at very low values of Gr Re 2 . Mechanisms suggested by Petukhov and Polyakov explain these effects. The results should be useful in the development and validation of turbulence models for such flows.

Pressure Field and Main-Stream Gas Ingestion in a Rotor-Stator Disk Cavity

The time-average and unsteady static pressure fields, and the velocity field were measured in a rotor-stator disk cavity and its main-stream gas path. Ingestion of main gas into the disk cavity was also measured. Some CFD simulations were performed. The time-average static pressure was circumferentially periodic (following the vane pitch) in the main-stream gas path and circumferentially uniform in the cavity. The unsteady pressure in the stationary frame of reference was periodic, following the passing of rotor blades, both in the main gas path and in the disk cavity. In the main gas path, the circumferential asymmetry amplitude of the time-average static pressure and the pressure unsteadiness amplitude were of the same order of magnitude. Steady CFD simulations were unable to predict ingestion in the cases where ingestion was detected by tracer gas measurement. It appears that simulations will need to be unsteady as well as three-dimensional to be able to predict ingestion correctly.© 2001 ASME

Velocity and temperature fields in turbulent liquid flow through a vertical concentric annular channel

Abstract Velocity and temperature fields are measured in turbulent upflow of liquid Refrigerant-113 through a vertical concentric annular channel. Data are reported in the Reynolds number range of 18000–50300 and inner wall heat flux range of 0–30000 W m−2. The velocity field measurements are generally successful whereas some difficulty is encountered in the measurement of single-point correlation between turbulent velocity and temperature fluctuations.

Stochastic characteristics of vapor fraction and wall pressure fluctuations in boiling flows

Abstract An experimental investigation of the statistical character of boiling flows has been carried out. Two flow field variables, viz. static pressure fluctuations at the test section outer wall and instantaneous chordal-average vapor fraction, were studied in vertical up-flow through concentric annular test sections. Matched piezo-electric pressure transducers were used for the pressure fluctuation measurements, and a linearized dual-beam X-ray system was used for the vapor fraction measurements. Steady state (mean) thermal-hydraulic conditions in the test section were determined by an analytical model and verified to a certain extent by capacitance probe vapor volume fraction measurements. A wide range of local conditions with flow regimes ranging from subcooled bubbly to saturated churn turbulent-slug-annular were investigated. The wall static pressure fluctuation results include: (i) intensity (RMS value). (ii) probability density function: and (iii) autopower spectral density function. The chordal-average vapor fraction results include: (i) probability density function: and (ii) autopower spectral density function. The magnitude of coherence between wall static pressure fluctuations and chordal-average vapor fraction fluctuations are presented as well. It is suggested that diagnosis of flow regimes on the basis of the statistical properties of the two variables studied should be possible.