Optical Probe Data Research Articles

Best practices for velocity estimations in highly aerated flows with dual-tip phase-detection probes

Dual-tip phase-detection probes can be used to measure flow properties in gas-liquid flows. Traditionally, time-averaged interfacial velocities have been obtained through cross-correlation analysis of long time-series of phase fraction signals. Using small groups of detected particles, a recently developed adaptive window cross-correlation (AWCC) technique enables the computation of pseudo-instantaneous interfacial velocities and turbulence quantities in highly aerated flows, albeit subject to some smoothing which is due to the use of a finite window duration. This manuscript provides guidance on the selection of optimum processing parameters for the AWCC technique, additionally addressing shortcomings such as velocity bias correction and extrapolation of turbulence levels to single particles. The presented technique was tested for three self-aerated flows: smooth and rough-wall boundary layers (tunnel chute and stepped spillway), as well as breaking shear layer flows of a hydraulic jump. Robust estimations of mean velocities and velocity fluctuations were obtained for all flow situations, either using dual-tip conductivity or fiber optical probe data. The computation of integral time scales and velocity spectra is currently limited by the data rate and must be treated with caution.

Identification of flow regime in a bubble column reactor with a combination of optical probe data and machine learning technique

In the present work, a data-driven model for identification of flow regime in a bubble column is developed by combining data from optical probe technique and machine learning. Optical probe data from previous work was combined with new data in the present work to expand the database for model development. A novel methodology for determination of two key parameters from the optical probe signal, bubble time and characteristic time of the signal, is presented. The significance of these two parameters is that they contain rich information on operating flow regime in the bubble column. A map of these two parameters for various operating conditions is created, showing points belonging to identical flow regime lie in a cluster. A machine learning methodology based on support vector analysis was developed to identify flow regime using map developed in this work. This approach was able to uniquely classify flow regimes for various experimental conditions on single map, which is the highlight of this work.

Generation and evaluation of an artificial optical signal based on X-ray measurements for bubble characterization in fluidized beds with vertical internals

The performance of fluidized bed reactors strongly depends on the bubble behavior, for which reason knowledge concerning the bubble properties is important for modeling and reactor optimization. X-ray measurements can be used to characterize bubbles within the cross-section of a fluidized bed on a laboratory scale, but cannot easily be extended to hot, pressurized large scale plants. For future measurements at hot conditions in a fluidized bed methanation reactor, we have developed an optical probing system that can be employed under these conditions. However, optical sensors are only able to investigate the local fluidization patterns at a defined position in the bed. The objective of this study is to characterize differences in bubble properties between local optical measurements and an X-ray tomography method that is able to detect bubbles over the entire cross-section. Therefore, an artificial optical signal is created out of existing hydrodynamic X-ray measurement data obtained at a cold flow model of a pilot scale methanation reactor. The determined bubble properties of both methods (i.e. evaluation of the derived artificial optical probe signal and image reconstruction based on the evaluation of original X-ray tomographic data) are compared with regard to the bubble rise velocity and the bubble size (for the X-ray method) or pierced chord length (for the optical evaluation method), respectively. The comparison shows that for the evaluation of the optical probe data, statistical effects have to be considered carefully. The detected mean chord length of the optical method does not immediately correspond to the mean bubble size determined by the X-ray method. Moreover, also differences regarding the bubble rise velocity were detected for some fluidization states. The reason for the discrepancies between both methods could be identified and corrected, amongst others by means of a Monte Carlo simulation in which rising bubbles in a fluidized bed were simulated and characterized by a local virtual optical sensor.

Two-phase structure above hot surfaces in jet impingement boiling

Jet impingement boiling is very efficient in cooling of hot surfaces as a part of the impinging liquid evaporates. Several studies have been carried out to measure and correlate the heat transfer to impinging jets as a function of global parameters such as jet subcooling, jet velocity, nozzle size and distance to the surface, etc. If physically based mechanistic models are to be developed, studies on the fundamentals of two-phase dynamics near the hot surface are required. In the present study the vapor–liquid structures underneath a subcooled (20 K) planar (1 mm × 9 mm) water jet, impinging the heated plate vertically with a velocity of 0.4 m/s, were analyzed by means of a miniaturized optical probe. It has a tip diameter of app. 1.5 μm and is moved toward the plate by a micrometer device. The temperature controlled experimental technique enabled steady-state experiments in all boiling regimes. The optical probe data provides information about the void fraction, the contact frequencies and the distribution of the vapor and liquid contact times as a function of the distance to the surface. The measured contact frequencies range from 40 Hz at the onset of nucleate boiling to nearly 20,000 Hz at the end of the transition boiling regime. Due to condensation in the subcooled jet vapor disappears at a distance to the surface of app. 1.2 mm in nucleate boiling. This vapor layer becomes smaller with increasing wall superheat. In film boiling a vapor film thickness of 8 ± 2 μm was found.

Heat transfer characteristics and mechanisms along entire boiling curves under steady-state and transient conditions

The paper presents a survey of results found by the authors and their groups during recent years. An experimental technique for precise and systematic measurements of entire boiling curves under steady-state and transient conditions has been developed. Pool boiling experiments for well wetting fluids and fluids with a larger contact angle (FC-72, isopropanol, water) yield single and reproducible boiling curves if the system is clean. However, even minimal deposits on the surface change the heat transfer characteristic and shift the boiling curve with each test run. The situation is different under transient conditions: heating and cooling transients yield different curves even on clean surfaces. Measurements with micro-optical probes give an insight in the two-phase dynamics above the heating surface in the different boiling regimes. Temperature signals from microthermocouples at the surface and the optical probe data lead to some conclusions on the physics of boiling in different regimes such as the macrolayer configuration, the dry-spot size and dynamics, the nucleation site density etc. The high density of active nucleation sites in the region between fully developed nucleate boiling and critical heat flux (CHF) and the resulting highly turbulent two-phase boundary layer let us conclude that the strong increase of heat flux in heating transients and vice versa in a cooling process is mainly due to the intensive two-phase convection heat transfer from the wall to the bulk and not due to inertia effects in the two-phase structure dynamics. The latter is not significantly affected by temperature transients. An interfacial-area-density model is proposed. It needs data from the optical probes and enables the prediction of entire boiling curves by employing only one fitting parameter. Furthermore the concept of a reaction–diffusion model is presented to predict CHF. Here the triggering of CHF is due to an instability of dry spots on the heating surface. Data from microthermocouples and optical probes are needed to evaluate this model. Many aspects of the extremely complex mechanisms of boiling are still not sufficiently understood. The problems should be tackled from both the experimental and the theoretical end and both approaches should be closely linked.

Experimental studies of boiling mechanisms in all boiling regimes under steady-state and transient conditions

The paper presents a summary of results found in the first author's laboratories during recent years. First an experimental technique suitable for precise and systematic measurements of entire boiling curves under steady-state and transient conditions is described. The experimental results for pool boiling of well wetting fluids and fluids with a larger contact angle (FC-72, isopropanol, water) show no evidence of a hysteresis in the transition region if the heater surface is clean. For a surface with contamination, however, the boiling curves are not reproducible. The situation is different under transient conditions: heating and cooling transients yield different curves. Boiling experiments with a water jet impinging vertically on a hot surface show similar boiling mechanisms as in pool or forced convective boiling except for the transition region, but the heat fluxes are significantly larger. In the transition region very large heat fluxes are observed until a rather abrupt transition to film boiling at very high wall superheats. Measurements with a micro-optical probe give a detailed insight in the behavior of the macrolayer in the different boiling regimes. Temperature signals from microthermocouples very near the heater surface and the optical probe data lead to some conclusions on the physics of boiling in the different boiling regimes. In particular it was found that convective effects in the boundary layer are probably responsible for the deviations between transient and steady-state boiling heat transfer.

THE EFFECT OF REFRACTIVE INDEX ON SIZE DISTRIBUTIONS AND LIGHT SCATTERING COEFFICIENTS DERIVED FROM OPTICAL PARTICLE COUNTERS

The effect of refractive index on particle size distributions measured by optical particle counters is examined. Similar to previous investigations, it is found that optical counters undersize ambient particles because the refractive index of these particles is generally lower than that of the latex particles commonly used for the calibration of optical counters. The maximum undersizing is found to occur when particle sizes are comparable to the wavelength of light used in the measurement. A new approach for modeling the effect of refractive index on the sizing of optical counters is presented. Previously derived optical response functions are compared and a generalized formulation is proposed which includes the existing response functions as special cases. Algorithms are presented for correcting size distributions measured by optical counters for the difference between the refractive index of ambient and calibration particles. Data collected by a Passive Cavity Aerosol Spectrometer (PCASP) and by an integrating nephelometer are compared. Light scattering coefficients calculated from the optical probe data uncorrected for the effect of refractive index differ from those measured by the integrating nephelometer by a factor of 2. An iterative procedure that adjusts the PCASP-measured size distribution for the effect of refractive index is used to derive the best agreement between calculated and observed light scattering coefficients. The refractive indices of aerosols at wavelength of 0.45 μ m that best fit the data vary between 1.3 and 1.5, with an average of 1.41. The relative importance of the underestimation of light scattering coefficients calculated from the PCASP-measured size distributions due to the refractive index and the size truncation effect are evaluated. The former is found to be more important than the latter. Implications of this study for addressing aerosol shortwave radiative forcing and potential uncertainties relevant to this study are discussed.

Genotype and treatment biases in estimation of carcass lean of swine.

Carcasses of 181 barrows, representing five genotypes, 1) H x HD, 2) SYN, 3) HD x L[YD], 4) L x YD, and 5) Y x L (H = Hampshire, D = Duroc, SYN = synthetic terminal sire line, L = Landrace, and Y = Yorkshire), and two levels of ractopamine (RAC) treatment (0 and 20 ppm) were completely dissected and the data were used to examine genotype and treatment (RAC) biases in estimation of fat-standardized lean weight and to evaluate accuracies and precisions realized by use of equations based on variables derived from different technologies. Independent variables used to establish regression equations represented technologies of direct carcass measurements, optical probe data, TOBEC (total body electrical conductivity) readings, and dissected (DHMLN) and fat-standardized (FSHMLN) ham lean. Genotype bias existed when any equation from a single technology was used and was minimized by combining FSHMLN with one TOBEC reading, carcass length, and the probe measurement of 10th rib fat depth. Large RAC biases appeared when equations from direct carcass measurements or optical probe data were used and were minimized by an equation using either DHMLN or FSHMLN. A practical equation with relatively high R2 value and small genotype and RAC biases were developed by combining TOBEC readings with direct carcass measurements of 10th rib fat depth and warm carcass weight.