Particle Size Velocity Research Articles

Development and Evaluation of the Raindrop Size Distribution Parameters for the NASA Global Precipitation Measurement Mission Ground Validation Program

AbstractThe National Aeronautics and Space Administration Global Precipitation Measurement (GPM) mission ground validation program uses dual-polarization radar moments to estimate raindrop size distribution (DSD) parameters, the mass-weighted mean drop diameter Dmass, and normalized intercept parameter NW, to validate the GPM Core Observatory–derived DSD parameters. The disdrometer-based Dmass and NW are derived through empirical relationships between Dmass and differential reflectivity ZDR, and between NW, reflectivity ZH, and Dmass. This study employs large datasets collected from two-dimensional video disdrometers (2DVD) during six different field studies to derive the requisite empirical relationships. The uncertainty of the derived Dmass(ZDR) relationship is evaluated through comparisons of 2DVD-calculated and ZDR-estimated Dmass, where ZDR is calculated directly from 2DVD observations. Similarly, the uncertainty of the NW(ZH, Dmass) relationship is evaluated through 2DVD-calculated and Dmass and ZH-estimated NW, where Dmass and ZH are directly calculated from 2DVD observations. This study also presents the sensitivity of Dmass(ZDR) relationships to climate regime and to disdrometer type after developing three additional Dmass(ZDR) relationships from second-generation Particle Size Velocity (PARSIVEL2) disdrometer (P2) observations collected in the Pacific Northwest, in Iowa, and at Kwajalein Atoll in the tropical Pacific Ocean. The application of P2-derived Dmass(ZDR) relationship based on precipitation in the northwestern United States to P2 observations collected over the tropical ocean resulted in the highest error among comparisons of the three datasets.

Establishing fluidization parameters of different size of coal ash particles in bubbling fluidized bed

The Fluidization behavior of coal ash particles was studied to establish fluidization parameters in a cylindrical bubbling fluidized bed. A series of experiments were carried out with different sizes of irregular coal ash particles ranging from 1 mm to 3 mm at different bed heights at atmospheric pressure and room temperature (ca. 28 °C). Fluidization regimes of all selected coal ash particles were studied by varying flow rates of fluidization medium i.e., air. The change in bed height and pressure drop across the bed was carefully analyzed to see the effects of particle size and superficial gas velocity during fluidization. The minimum fluidization velocity (Umf), minimum bubbling velocity (Umb) and fluidization index for different sizes of ash particles in cold conditions were calculated. The validation of experimental values was supported by theoretical calculations. It was observed that the Umb is 1.5–1.8 times higher than the Umf at different bed heights (Hbed). No substantial deviation was found in Umf with the variations of Hbed at a particular mean particle diameter (dp). Finally, the fluidization index between 1.58 and 1.78 for coal ash particles of Geldart-D type (dp> 1000 µm) shows an ideal condition for fluidization.

Validation of GPM Precipitation Products by Comparison with Ground-Based Parsivel Disdrometers over Jianghuai Region

In this study, we evaluated the performance of rain-retrieval algorithms for the Version 6 Global Precipitation Measurement Dual-frequency Precipitation Radar (GPM DPR) products, against disdrometer observations and improved their retrieval algorithms by using a revised shape parameter µ derived from long-term Particle Size Velocity (Parsivel) disdrometer observations in Jianghuai region from 2014 to 2018. To obtain the optimized shape parameter, raindrop size distribution (DSD) characteristics of summer and winter seasons over Jianghuai region are analyzed, in terms of six rain rate classes and two rain categories (convective and stratiform). The results suggest that the GPM DPR may have better performance for winter rain than summer rain over Jianghuai region with biases of 40% (80%) in winter (summer). The retrieval errors of rain category-based µ (3–5%) were proved to be the smallest in comparison with rain rate-based µ (11–13%) or a constant µ (20–22%) in rain-retrieval algorithms, with a possible application to rainfall estimations over Jianghuai region. Empirical Dm–Ze and Nw–Dm relationships were also derived preliminarily to improve the GPM rainfall estimates over Jianghuai region.

Improvement in head loss characteristics of fine particulate coal-water suspension with addition of coarser particulate

ABSTRACT An experimental study is performed to examine the pressure drop characteristics during the flow of coal-water slurry suspension using a pilot-plant test loop. The effect of slurry concentration, particle size, and flow velocity on the pressure drop characteristics during the transportation through a horizontal pipeline has been investigated. The present study considers both the unimodal and bimodal particle size distributions for the preparation of coal-water slurry. The pressure drop characteristics for both unimodal and bimodal coal-water slurry are analyzed for a wide variety of slurry concentrations ranging from 30% to 60%, which covers from low to high concentrations. The pressure drop characteristics are also analyzed for different flow velocities ranging from 2 to 5 m/s. It is found that the amount of pressure drop during the flow of coal-water slurry suspension in horizontal pipeline is the function of solid concentration, particle size, and flow velocity. It is also found that the bimodal coal-water slurry significantly reduces the pressure drop during the flow as compared to unimodal coal-water slurry. The maximum reduction in pressure drop during the flow is observed by bimodal coal-water slurry prepared by adding 30% of coarser particles in finer coal particles.

Characteristics of Summer Season Raindrop Size Distribution in Three Typical Regions of Western Pacific

AbstractRaindrop size distribution (DSD) measurements were taken with an onboard OTT Particle Size Velocity (Parsivel) disdrometer over the western Pacific during a marine survey from June to July 2014. Three subregions named south western Pacific (SWP), west western Pacific, and north western Pacific were separated for a comparative study of the variability of DSD. In addition to disdrometer data, FengYun‐2E, Moderate Resolution Imaging Spectroradiometer, National Centers for Environmental Prediction Global Final Analysis, and radiosonde data sets are used to illustrate the dynamical and microphysical characteristics associated with summer season rainfall of western Pacific. The DSD characteristics of six different rain rates and two rain types (convective and stratiform) were studied. Histograms of normalized intercept parameter log10(Nw) and mass‐weighted mean diameter Dm indicated largest log10(Nw) values in west western Pacific, while largest Dm values in south western Pacific, and the convective clusters in three regions could be identified between maritime like and continental like. The constrained relations between shape μ and slope Λ, Nw, and Dm of gamma DSDs are derived. An inverse relation of the coefficients and exponents of Z‐ARb for convective rain were found in three regions. The R (ZH, ZDR) estimator is proved to be more accurate than Z‐R relation algorithm. And the empirical relations between Dm and radar reflectivity factor in the Ku and Ka bands are also derived to improve the rainfall retrieval algorithms in the open sea of Pacific. Furthermore, the possible causative mechanisms for the significant DSD variability in three regions were investigated with respect to convective intensity, raindrop evaporation, and other meteorological variables.

Statistical characteristics of raindrop size distribution in south China summer based on the vertical structure derived from VPR-CFMCW

The raindrop size distribution (DSD) characteristics during the precipitation season are analyzed using data collected by an OTT Particle Size Velocity (Parsivel) disdrometer and a Vertical Pointing Radar with C-band Frequency Modulation Continuous Wave (VPR-CFMCW) technology. The two datasets were collected at the same site in Longmen, Guangdong Province, which is the precipitation center of south China, from June to July in 2016 and 2017. We evaluate different fitting methods for the gamma model function and choose a nonlinear least-squares method to fit DSD. Based on the radar reflectance obtained by VPR-CFMCW, the precipitating clouds that occur during the summer precipitation season in south China are classified into four types (i.e., convective, stratiform, mixture, and shallow). The characteristic parameters and the gamma model parameters of different precipitation types are compared. Avoiding the limitations of rainfall classification at the surface, the new classification quantifies the characteristics of mixture and shallow precipitation. The results show that the stratiform precipitation makes up 43.1% of the summer precipitation process in south China, and the contribution of convective precipitation to total rainfall is 62.7%. The precipitation parameters of the four types of precipitation, such as the rain rate (R), the mass-weighted mean diameter (Dm), the radar reflectance (Z), and the liquid water content (LWC), follow the pattern: convective > mixture > stratiform > shallow. The DSD characteristics of the four precipitating cloud types are investigated. For the DSD of convective and mixture precipitation, the spectra width is similar but the rain drop concentration of the mixture is smaller. For the DSD of stratiform and shallow clouds, the rain drop concentrations are similar, but the spectra width of the shallow clouds are smaller. In addition, the relationships between μ-Λ, Dm-Nw, Dm-R, and Z-R are obtained. These new relationships will help improve the accuracy of precipitation estimation and deepen the understanding of the characteristics of surface precipitation microphysical parameters for different types of precipitating clouds in south China.

Measurement of abrasive particle velocity and size distribution in high pressure abrasive slurry and water micro-jets using a modified dual disc anemometer

In both abrasive water jet (AWJ) and high pressure abrasive slurry jet (HASJ) machining, garnet abrasive particles are mixed with water and the resulting jet can contain particles travelling with a wide range of velocities. Previous investigations have suggested that a dual disc anemometer (DDA) can be used to provide an assessment of the average particle velocity within the jets. This paper presents an in-depth analysis of the DDA test by considering the distribution and size of impact craters on the recording disc, and by modeling the size of the slurry slug incident to it. Procedures are presented to assess the accuracy of the determined average velocity, and to determine the histogram of particle velocities within the jet. A modification to the anemometer is introduced which also allows determination of the particle size-velocity correlation. Results are presented for both HASJ and AWJ under typical process conditions, and compared to existing analytical models of abrasive particle velocity from the literature, with good agreement.

Comparative Evaluation of the OTT PARSIVEL2 Using a Collocated Two-Dimensional Video Disdrometer

AbstractThe performance of the OTT second-generation Particle Size Velocity (PARSIVEL2) laser weather sensor is evaluated by comparing it with a collocated two-dimensional video disdrometer (2DVD) and rain gauges using data collected over a total of 36 rain events. A comparison of raindrop size distributions (DSDs) between the 2DVD and two PARSIVEL2 reveals good agreement for weak rainfall rates below approximately 10 mm h−1 and for midsize drops with diameters between 0.6 and 4.0 mm irrespective of rainfall rates, whereas the PARSIVEL2 produces overestimations of large drops with diameters above 4 mm during heavy rainfall above approximately 20 mm h−1. The resultant DSD parameters of the PARSIVEL2 present overestimations of the mean diameter Dm in the normalized gamma function and the maximum drop diameter Dmax, and underestimations of the intercept parameter Nw and total number of drops NT. Furthermore, how the characteristics of DSDs from the PARSIVEL2 affect the polarimetric radar variables, such as differential reflectivity ZDR and specific differential phase KDP, is examined, as well as how these characteristics affect empirical relations required in radar hydrometeorological applications such as quantitative rainfall estimations. Based on these examinations, it can be concluded that the OTT PARSIVEL2 still produces overestimations of large drops and underestimations of small drops during heavy rainfall, similar to older models of PARSIVEL, despite significant improvements to the PARSIVEL2 system, and furthermore that the uses of PARSIVEL2 measurements can act as a source of error in radar hydrometeorological applications such as radar rainfall estimations.

Comparison of Raindrop Size Distributions in a Midlatitude Continental Squall Line during Different Stages as Measured by Parsivel over East China

AbstractThe characteristics of raindrop size distributions (DSDs) during a midlatitude continental squall line on 30 July 2014 in east China are studied, and the different life stages are observed by OTT second-generation Particle Size Velocity (Parsivel2) disdrometers at Chuzhou during the mature stage and Nanjing during the declining stage. The observed rainfall is classified into convective line, transition, and stratiform regions based on the structure of the radar reflectivity Z and rainfall intensity R. The results show that the DSD characteristics of the different precipitation types and different squall-line stages are very different. The convective center has the largest number concentration and quantity of large drops corresponding to the highest rain rate; the rain rates in the trailing edge and stratiform regions are similar, although a lower concentration of small drops is present in the latter. Between the two stages, the drop size and number concentration for the convective center decrease, although the leading edge during the declining stage has more numerous larger drops; the number concentration is similar in the stratiform rainfall, but the drops become much smaller. For the normalized distribution, the scaled spectra for the convective center are closer to an exponential distribution, and the μ value during the declining stage is larger than that during the mature stage for the stratiform region and similar during both stages for the convective center. The declining stage has a larger exponent b and smaller coefficient A in the Z–R relationship based on fits for the entire dataset. Moreover, the R(ZH, ZDR) estimator is more accurate than that when using the Z–R relation algorithm.

Impacts of Instrument Limitations on Estimated Raindrop Size Distribution, Radar Parameters, and Model Microphysics during Mei-Yu Season in East China

AbstractInstrumentation limitations on measured raindrop size distributions (DSDs) and their derived relations and physical parameters are studied through a comparison of the DSD measurements during mei-yu season in east China by four collocated instruments, that is, a two-dimensional video disdrometer (2DVD), a vertically pointing Micro Rain Radar (MRR), and two laser-optical OTT Particle Size Velocity (PARSIVEL) disdrometers (first generation: OTT-1; second generation: OTT-2). Among the four instruments, the 2DVD provides the most accurate DSD and drop velocity measurements, so its measured rainfall amount has the best agreement with the reference rain gauge. Other instruments tend to miss more small drops (D < 1 mm), leading to inaccurate DSDs and a lower rainfall amount. The low rainfall estimation becomes significant during heavy rainfall. The impacts of instrument limitations on the microphysical processes (e.g., evaporation and accretion rates) and convective storm morphology are evaluated. This is important especially for mei-yu precipitation, which is dominated by a high concentration of small drops. Hence, the instrument limitations need to be taken into account in both QPE and microphysics parameterization.

Computer simulation of coal gasification in a full scale plant

ABSTRACTThis paper deals with the modeling and simulation of air–steam gasification of coal using Aspen Plus process simulator. The model is able to predict the gas products, tar concentration, and char yield where the prediction accuracy of the presented model has been validated against the experimental measurements and found to be in a good agreement. The effect of coal particle size and minimum fluidization velocity (Umin) in air–steam gasification is explored through key operating parameters such as reaction temperature, equivalence ratio, and steam/coal ratio. Maximum hydrogen yield of 50.5 g/kg coal was obtained at a temperature of 650°C and steam/coal ratio of 0.7. Results were also showed that the particle size plays a key role in the process; smaller particles favor higher hydrogen yield.

Classification of rain types using drop size distributions and polarimetric radar: Case study of a 2014 flooding event in Korea

To classify precipitation types as either convective or stratiform, drop size distributions (DSDs) measured by the Parsivel (PARticle size VELocity) instrument, and DSD parameters including median volume diameter (D0) and normalized number concentration (Nw) retrieved by S-band polarimetric radar (BSL), were analyzed for a heavy rainfall event that occurred in southern Korea on 25 August 2014. The rainfall system was clearly identified as stratiform or convective rain at various times of day, at measurement sites at Changwon and Busan. New rainfall classification lines were derived from the Parsivel and BSL data, and were compared with existing classification methods based on climatological rainfall data.The classification methods using logNw-D0, logN0-rainrate, and slope-rainrate domain proposed in previous two studies performed well when applied to the new data if the slope and/or intercept values were changed. Another method using logN0-slope domain was not possible to classify the precipitation types well in the study area, as the best-fit line could not be obtained. The average measured D0 and Nw values obtained from polarimetric radar were compared with climatological precipitation data, measured in both the tropics and mid-latitudes. And new separation line was obtained for the rainfall at the southern part of Korea.

Analysis on Erosion of Pipe Bends Induced by Liquid-solid Two-phase Flow

The effect of particle-parameters of a liquid-solid two phase flow on the erosion rate of different sections for pipebends was analyzed by means of computational fluid dynamics (CFD) in terms of flow rate, particle size and particle velocity etc., while the trajectories of particles were calculated by Lagrange method. Then the relevant erosion mechanism of pipe wall may be acquired by considering the known particle collision model. The results show that: (1) the vulnerable areas mainly exist on the sidewall, as well as the outermost side of the junction of downstream straight pipe and elbow; (2) the change of Stokes number can cause shift of the serious erosion area, whilst not the entire sidewall of the junction of downstream straight pipe and elbow will be subjected to serious erosion.

Raindrop Size Distribution and Rain Characteristics during the 2013 Great Colorado Flood

Abstract Drop size distributions observed by four Particle Size Velocity (PARSIVEL) disdrometers during the 2013 Great Colorado Flood are used to diagnose rain characteristics during intensive rainfall episodes. The analysis focuses on 30 h of intense rainfall in the vicinity of Boulder, Colorado, from 2200 UTC 11 September to 0400 UTC 13 September 2013. Rainfall rates R, median volume diameters D0, reflectivity Z, drop size distributions (DSDs), and gamma DSD parameters were derived and compared between the foothills and adjacent plains locations. Rainfall throughout the entire event was characterized by a large number of small- to medium-sized raindrops (diameters smaller than 1.5 mm) resulting in small values of Z (<40 dBZ), differential reflectivity Zdr (<1.3 dB), specific differential phase Kdp (<1° km−1), and D0 (<1 mm). In addition, high liquid water content was present throughout the entire event. Raindrops observed in the plains were generally larger than those in the foothills. DSDs observed in the foothills were characterized by a large concentration of small-sized drops (d < 1 mm). Heavy rainfall rates with slightly larger drops were observed during the first intense rainfall episode (0000–0800 UTC 12 September) and were associated with areas of enhanced low-level convergence and vertical velocity according to the wind fields derived from the Variational Doppler Radar Analysis System. The disdrometer-derived Z–R relationships reflect how unusual the DSDs were during the 2013 Great Colorado Flood. As a result, Z–R relations commonly used by the operational NEXRAD strongly underestimated rainfall rates by up to 43%.

Experimental Study on Crushing Characteristic of Coal and Gangue under Impact Load

ABSTRACTIn order to analyze crushing characteristics of coal and gangue and its influence on separation effect, impact crushing experiments for coal and gangue were conducted. Crushing characteristics of coal and gangue under impact load were expressed by crushing ratio and crushing probability. Experimental results showed that crushing ratios of coal and gangue decreased with an increase of particle size and impact velocity. As the impact velocity was between 5–8 m/s and the initial particle size was between 50–100 mm, the particle size of coal obtained after crushing was between 35 mm and 45 mm. Crushing probabilities of coal and gangue increased with the impact velocity, but it was far less than that of coal under the same conditions. The lost coal rate and the mixed gangue rate were used to represent the separation effect. When the impact velocity was 7.5 m/s, the lost coal rate and the mixed gangue rate for the Xiezhuang mine coal sample were 18.285% and 25.723%, respectively. The separation effect predicted by Back Propagation Artificial Neural Network (BP-ANN) showed that the lost coal rate and the mixed gangue rate for the Xiezhuang mine coal at an impact velocity of 8.5 m/s were 3.68% and 33.35%, respectively.

Decadal Variation in Raindrop Size Distributions in Busan, Korea

This paper investigated the variability of raindrop size distributions (DSDs) in Busan, Korea, using data from two different disdrometers: a precipitation occurrence sensor system (POSS) and a particle size velocity (Parsivel) optical disdrometer. DSDs were simulated using a gamma model to assess the intercomparability of these two techniques. Annual rainfall amount was higher in 2012 than in 2002, as were the annually averagedDm(which was 0.1 mm greater in 2012) and the frequency of convective rain. Severe rainfall (greater than 20 mm h−1) occurred more frequently and with a largerDmin 2012. The values ofDmfrom July, August, and December, 2012, were much greater than from other months when compared with 2002. Larger raindrops contributed to the higher rain rates that were observed in the morning during 2012, whereas relatively smaller raindrops dominated in the afternoon. These results suggest that the increase in raindrop size that has been observed in Busan may continue in the future; however, more research will be required if we are to fully understand this phenomenon. Rainfall variables are highly dependent on drop size and so should be recalculated using the newest DSDs to allow more accurate polarimetric radar rainfall estimation.

Intercomparison of Solid Precipitation Derived from the Weighting Rain Gauge and Optical Instruments in the Interior Qinghai-Tibetan Plateau

Due to the light precipitation and strong wind in the cold season, it is hard to get credible solid precipitation on the Qinghai-Tibetan plateau (QTP). To address this issue, two kinds of optical instruments, the Thies Laser Precipitation Monitor (LPM) and OTT laser-optical Particle Size Velocity (Parsivel), were used on QTP. The measured precipitation was compared with the precipitation derived from Geonor T-200B precipitation gauge (Geonor). The results showed that Geonor was hard to catch light precipitation (precipitation amount was less than 1 mm during a single event) when the wind speeds were higher than 3.5 m/s. Even when the wind speeds were smaller than 3.5 m/s, about 44% of such light precipitation events were not recorded by Geonor. The optical instruments had much better performance in recording light precipitation. Three methods were used to correct Geonor measurements of daily solid precipitation and the corrected values were set as reference for assessing the performance of LPM and Parsivel; the results showed that LPM had good performance in measuring the solid precipitation but Parsivel overestimated the precipitation amount. Methods for correcting Geonor’s hourly solid precipitation and recalculating Parsivel’s solid precipitation amount were also proposed in this paper.

Evaluation of the New Version of the Laser-Optical Disdrometer, OTT Parsivel2

Abstract A comparative study of raindrop size distribution measurements has been conducted at NASA’s Goddard Space Flight Center where the focus was to evaluate the performance of the upgraded laser-optical OTT Particle Size Velocity (Parsivel2; P2) disdrometer. The experimental setup included a collocated pair of tipping-bucket rain gauges, OTT Parsivel (P1) and P2 disdrometers, and Joss–Waldvogel (JW) disdrometers. Excellent agreement between the two collocated rain gauges enabled their use as a relative reference for event rain totals. A comparison of event total showed that the P2 had a 6% absolute bias with respect to the reference gauges, considerably lower than the P1 and JW disdrometers. Good agreement was also evident between the JW and P2 in hourly raindrop spectra for drop diameters between 0.5 and 4 mm. The P2 drop concentrations mostly increased toward small sizes, and the peak concentrations were mostly observed in the first three measurable size bins. The P1, on the other hand, underestimated small drops and overestimated the large drops, particularly in heavy rain rates. From the analysis performed, it appears that the P2 is an improvement over the P1 model for both drop size and rainfall measurements. P2 mean fall velocities follow accepted terminal fall speed relationships at drop sizes less than 1 mm. As a caveat, the P2 had approximately 1 m s−1 slower mean fall speed with respect to the terminal fall speed near 1 mm, and the difference between the mean measured and terminal fall speeds reduced with increasing drop size. This caveat was recognized as a software bug by the manufacturer and is currently being investigated.

Simulation of Multi‐Stage Particle Classification in a Zigzag Apparatus

AbstractThe separation efficiency of a pilot‐scale zigzag apparatus is investigated numerically using computational fluid dynamics simulations and discrete particle modeling in a coupled manner. The effects of various process variables, like particle size and air flow velocity, and of turbulence models were analyzed. The resulting changes concerning the process performance expressed by separation function and sharpness are discussed. Moreover, the residence time distribution was found to differ for fine and coarse particle discharges. Small particles are easily carried away by the fluid and respond immediately to almost every change in flow velocity. Therefore, they are affected by vortices, which increase their residence times compared to bigger particles.

Detecting Sudden Changes in Fluidization by Wall Vibration

Vibrations of fluidized bed walls reflect the nonlinear characteristics of bed hydrodynamics in gas–solid fluidized beds. Experiments were carried out in a lab-scale, two-dimensional fluidized bed operated at ambient conditions for three particle sizes, various gas velocities, three aspect ratios, and different probe locations. The S-statistic method, which is, in fact, the comparison of attractors of two dynamic signals in the state space, was used to determine de-fluidization condition in the bed. Different scenarios were tested to evaluate whether this method is able to detect changes in the hydrodynamics fluidized bed based on the bed vibrations, including change in the bed mass, particle size, and gas velocity. The results showed that this method is capable of detecting the de-fluidization state in the bed as a result of changes in gas velocity, particle size, and bed mass. However, an important factor is the location of probe, which can dramatically affect the capability of this method for detecting the de-fluidization state.