Local Pressure Drop Research Articles

Evolution of the Pressure Profile during the Gelation and Restart of a Model Waxy Crude Oil

When pipelines are used to transport waxy crude oils in cold environments, the pipelines may become plugged with wax gels during periods of flow shutdown. It is a common practice to dislodge these gels by application of upstream pressure (“gel restart”), because the gel will fracture when the wall shear stress, proportional to the local axial pressure drop, exceeds the static yield stress of the gel. To better understand this process, we present results for the axial pressure profile measured as a function of time for a model waxy crude oil in a laboratory flow loop. Results are presented for both gels formed under ambient pressure and gels formed with a constant upstream pressure applied to minimize gas void formation. Gels formed in the latter case are more difficult to dislodge from the pipeline. The axial pressure profile in the gel just prior to a restart attempt depends upon the duration of the flow shutdown period. If a restart attempt is made soon after the oil gels, then large wall shear stresses arising from the axial pressure gradient are present in the gel. On the other hand, if the gel is aged for a long time prior to a restart attempt, then the axial pressure gradient approaches zero, a result that we attribute to a minute Darcy-type flow of the liquid components of the wax gel.

A102 固体高分子形燃料電池の流路内液滴形状の検討とガス流動解析(OS5 水素・燃料電池(1))

A network model has been proposed in order to predict the gas velocity distributions in a PEFC It analyzes the gas-velocity distributions depending on the local pressure drop which is affected by the gas-flow rate and the water accumulation Liquid drop form was observed in a PEFC by using neutron radiography, and classified into two models The one is small liquid drop accumulated near the rib corner, and the other is larger drop which is generated to be combined with small drops Numerical simulations were carried out for obtaining the local pressure drop caused by a liquid drop, and correlation equations were obtained for the pressure loss coefficient

Disintegration of materials by cavitating microjets

In the paper is presented an investigation of material disintegration by cavitating microjets. Cavitating microjet develops behind the micro-orifice at high flow speeds, when local pressure drop initiates a cavitation phenomenon. Described is a method and presented are selected results of experiments. Experiments were carried out with 2 micro-orifices at different flow conditions (cavitation number, distance between sample and micro-orifice). Experiments are based on flow visualisation as well as on a character of material displacement.

Measurement of subcooled boiling pressure drop and local heat transfer coefficient in horizontal tube under LPLF conditions

Horizontal flow is commonly encountered in boiler tubes, refrigerating equipments and nuclear reactor fuel channels of pressurized heavy water reactors (PHWR). Study of horizontal flow under low pressure and low flow (LPLF) conditions is important in understanding the nuclear core behavior during situations like LOCA (loss of coolant accidents). In the present work, local heat transfer coefficient and pressure drop are measured in a horizontal tube under LPLF conditions of subcooled boiling. Geometrical parameters covered in this study are diameter (5.5mm, 7.5mm and 9.5mm) and length (550mm, 750mm and 1000mm). The operating parameters varied are mass flux (450–935kg/m2s) and inlet subcooling (29°C, 50°C and 70°C). Infra-red thermography is used for the measurement of local wall temperature to estimate the heat transfer coefficient in single phase and two phase flows with water as the working medium at atmospheric pressure. Correlation for single phase diabatic pressure drop ratio (diabatic to adiabatic) as a function of viscosity ratio (wall temperature to fluid temperature) is presented. Correlation for pressure drop under subcooled boiling conditions as a function of Boiling number (Bo) and Jakob number (Ja) is obtained. Correlation for single phase heat transfer coefficient in the thermal developing region is presented as a function of Reynolds number (Re), Prandtl number (Pr) and z/d (ratio of axial length of the test section to diameter). Correlation for two phase heat transfer coefficient under subcooled boiling condition is developed as a function of boiling number (Bo), Jakob number (Ja) and Prandtl number (Pr).

어긋나기배열 직교류 열교환기의 열전달특성에 관한 연구

열교환기는 다수의 원관으로 구성하고 있기 때문에 원관 주위에서 국소열전달과 압력강하의 해석, 크기의 성능과 추산, 경제성으로 설계 시 중요한 역할을 한다. 본 연구에서는 어긋나기배열 직교류 열교환기에서 물의 온도 및 공기량 변화에 따른 대류열전달계수, 대수평균온도차, 압력손실 등을 고찰하기 위하여 실험 및 해석을 수행하였다. 본 열교환기는 관군이 5행 7열 어긋나기배열로서 구성하였으며, 실험 및 해석 조건은 물의 온도는 40℃∼65℃ 범위이고, 공기량은 5.0∼12.3㎥/s 범위이다. 그 결과로서 물의 온도 및 유량을 증가함에 따라 공기밀도가 감소하여 유속도 낮아지는 특성을 보여 레이놀즈수가 감소하고, 공기량 증대로 평균열전달계수가 증가하여 전열성능은 향상됨을 알 수 있었고, 압력손실도 증가 하였다. 그리고 해석결과로서는 열전달율의 경우는 약 8∼12%, 압력강하는 약 0.01∼7.5% 오차를 나타내어 본 연구의 적합성을 평가할 수 있었다.

Experimental and Numerical Investigation of the Gas Side Heat Transfer and Pressure Drop of Finned Tubes—Part II: Numerical Analysis

The heat transfer and pressure drop behavior of segmented circular and helical as well as solid finned tubes are investigated in a three-dimensional numerical study. The simulation is carried out using a finite volume method for calculating the steady-state temperature and flow field of the fluid as well as the temperature distribution of the tube material. For modeling the turbulence, the k-ε turbulence model based on the renormalization group theory (RNG) is used to resolve the near-wall treatment between adjacent fins. All simulations are performed in the Re range between 3500 ≤ Re ≤ 50,000. The influence of Reynolds number and fin geometry (segmented or solid and circular or helical) on the local and global averaged heat transfer and pressure drop was studied. A comparison between solid and segmented finned tube has shown that the heat transfer and pressure drop for the segmented finned tubes is higher. The numerical results are compared with experimental data.

Numerical investigation of single phase and two phase flow through thin orifices in horizontal pipes

Pressure drop due to single phase flow of water and two phase flow of air-water mixture through thin orifices in horizontal pipes have been numerically investigated. Two-phase computational fluid dynamics (CFD) calculations, using Eulerian–Eulerian model have been employed to calculate the pressure drop through orifices. The operating condition covers the gas and liquid superficial velocity ranges Vsg=0.3–4 m/s and Vsl=0.6–2 m/s, respectively. The local pressure drops have been obtained by means of extrapolation from the computed upstream and downstream linearized pressure profiles to the orifice section. Simulations for the single-phase flow of water have been carried out for local liquid Reynolds number (Re based on orifice diameter) ranging from 3×104 to 2×105 to obtain the discharge coefficient and the two-phase local multiplier, which when multiplied with the pressure drop of water (for same mass flow of water and two phase mixture) will reproduce the pressure drop for two phase flow through the orifice. The effect of orifice geometry on two-phase pressure losses has been considered by selecting eight different orifice plates with two area ratios (σ=0.73 and σ=0.54) and four different thicknesses (s/d = 0.025-0.59). The results obtained from numerical simulations are validated against experimental data from the literature and are found to be in good agreement. Keywords: Orifice, Pressure drop, Two phase flow, Computational fluid dynamics, Area ratio, Discharge coefficient

Single-Phase and Two-Phase Flow Through Thin and Thick Orifices in Horizontal Pipes

Two-phase flow pressure drops through thin and thick orifices have been numerically investigated with air–water flows in horizontal pipes. Two-phase computational fluid dynamics (CFD) calculations, using the Eulerian–Eulerian model have been employed to calculate the pressure drop through orifices. The operating conditions cover the gas and liquid superficial velocity ranges Vsg = 0.3–4 m/s and Vsl = 0.6–2 m/s, respectively. The local pressure drops have been obtained by means of extrapolation from the computed upstream and downstream linearized pressure profiles to the orifice section. Simulations for the single-phase flow of water have been carried out for local liquid Reynolds number (Re based on orifice diameter) ranging from 3 × 104 to 2 × 105 to obtain the discharge coefficient and the two-phase local multiplier, which when multiplied with the pressure drop of water (for same mass flow of water and two phase mixture) will reproduce the pressure drop for two phase flow through the orifice. The effect of orifice geometry on two-phase pressure losses has been considered by selecting two pipes of 60 mm and 40 mm inner diameter and eight different orifice plates (for each pipe) with two area ratios (σ = 0.73 and σ = 0.54) and four different thicknesses (s/d = 0.025–0.59). The results obtained from numerical simulations are validated against experimental data from the literature and are found to be in good agreement.

Analysis of outdoor temperature ranges for the operation of different ventilating modes in commercial buildings of China

Heat gains in commercial buildings in China are rather large compared to those of other public buildings, for occupant densities are usually high in such buildings. To match the requirements for both indoor environment and building energy conservation, ventilation systems, either mechanical ventilation, i.e., ventilating cooling (VC) or air-conditioning cooling (AC) systems, play important roles in maintaining good indoor air quality and thermal comfort. As the time durations for cooling and heating of AC systems in such buildings are different from other types of public buildings, temperature ranges for the application of different ventilation strategies all year round were obtained by using the local meteorological records in the present study. The results show that the time period in a whole year can be divided into five ranges according to the four outdoor critical temperatures for the operation of VC and AC systems with a heat recovery ventilator (HRV). The critical temperatures are mainly influenced by local climatic characteristics, occupant density, and pressure drop of the ventilation system, etc. Furthermore, energy effectiveness by employing different ventilation modes in commercial buildings with large internal gains was discussed and compared for four cities located in four different climatic zones of China.

Pressure drop during condensation of refrigerants in pipe minichannels

Pressure drop during condensation of refrigerants in pipe minichannelsThe present paper describes results of experimental investigations of pressure drop during the condensation of R134a, R404a and R407C refrigerants in pipe minichannels with internal diameter 0.31-3.30 mm. The results concern investigations of the mean and local pressure drop in single minichannels. The results of experimental investigations were compared with the calculations according to the correlations proposed by other authors. A pressure drop during the condensation of refrigerants is described in a satisfactory manner with Friedel and Garimella correlations. On the basis of the experimental investigations, the authors proposed their own correlation for calculation of local pressure drop during condensation in single minichannels.

Computational study of turbulent heat transfer for heating of water in a short vertical tube under velocities controlled

The steady-state turbulent heat transfer coefficients in a short vertical Platinum (Pt) test tube for the flow velocities (u = 4.11–41.07 m/s), the inlet liquid temperatures (Tin = 296.47–310.04 K), the inlet pressures (Pin = 810.40–1044.21 kPa) and the increasing heat inputs (Q0 exp(t/τ), exponential periods, τ, of 6.04–23.66 s) were systematically measured by an experimental water loop comprised of a multistage canned-type circulation pump with high pump head. Measurements were made on a 59.2 mm effective length and its three sections (upper, mid and lower positions), which were spot-welded four potential taps on the outer surface of the Pt test tube of a 6 mm inner diameter, a 69.6 mm heated length and a 0.4 mm thickness. Theoretical equations for turbulent heat transfer in a circular tube of a 6 mm in diameter and a 636 mm long were numerically solved for heating of water with heated section of a 6 mm in diameter and a 70 mm long by using PHOENICS code under the same condition as the experimental one considering the temperature dependence of thermo-physical properties concerned. The surface heat flux, q, and the average surface temperature, Ts,av, on the circular tube solved theoretically under the flow velocities, u, of 4.11–41.07 m/s were compared with the corresponding experimental values on heat flux, q, versus the temperature difference between average inner surface temperature and liquid bulk mean temperature, ΔTL [=Ts,av − TL, TL = (Tin + Tout)/2], graph. The numerical solutions of q and ΔTL are almost in good agreement with the corresponding experimental values of q and ΔTL with the deviations less than ±10% for the range of ΔTL tested here. The numerical solutions of local surface temperature, (Ts)z, local average liquid temperature, (Tf,av)z, and local liquid pressure drop, ΔPz, are within ±10% of the corresponding experimental data on (Ts)z, (Tf,av)z and ΔPz. The thickness of the thermal boundary layer, δTBL [=(Δr)out/2], and the non-dimensional thickness of thermal boundary layer, yTBL+[=(fF/2)0.5(ρluδVSL/μl)], for the turbulent heat transfer in a short vertical tube under velocities controlled are clarified based on the numerical solutions. It was confirmed in this study that authors’ steady-state turbulent heat transfer correlation based on the experimental data (Hata and Noda, 2008) can not only describe the experimental data of steady-state turbulent heat transfer but also the numerical solutions within ±10% difference for the wide ranges of temperature differences between heater inner surface temperature and liquid bulk mean temperature (ΔTL = 5–200 K) and flow velocities (u = 4.01–41.07 m/s).

Numerical Investigation of Single Phase and Two Phase Flow through Thin Orifices in Horizontal Pipes

Pressure drop due to single phase flow of water and two phase flow of air-water mixture through thin orifices in horizontal pipes have been numerically investigated. Two-phase computational fluid dynamics (CFD) calculations, using Eulerian-Eulerian model have been employed to calculate the pressure drop through orifices. The operating condition covers the gas and liquid superficial velocity ranges V sg =0.3-4 m/s and V sl =0.6-2 m/s, respectively. The local pressure drops have been obtained by means of extrapolation from the computed upstream and downstream linearized pressure profiles to the orifice section. Simulations for the single-phase flow of water have been carried out for local liquid Reynolds number (Re based on orifice diameter) ranging from 3×10 4 to 2×10 5 to obtain the discharge coefficient and the two-phase local multiplier, which when multiplied with the pressure drop of water (for same mass flow of water and two phase mixture) will reproduce the pressure drop for two phase flow through the orifice. The effect of orifice geometry on two-phase pressure losses has been considered by selecting eight different orifice plates with two area ratios (σ=0.73 and σ=0.54) and four different thicknesses (s/d = 0.025-0.59). The results obtained from numerical simulations are validated against experimental data from the literature and are found to be in good agreement.

Direct Observation of Cavitation Phenomenon and Hydrodynamic Lubrication Analysis of Textured Surfaces

When a textured ring rotates relatively against the other texture-free ring in a parallel thrust bearing, cavitation of liquid lubricant may occur in the divergent zones of the dimples or grooves on the textured surface due to local pressure drops. The Reynolds and Jakobsson–Floberg–Olsson (JFO) models are two widely used cavitation models in hydrodynamic lubrication theory, where the former lacks mass conservation while the latter enforces it. In order to investigate the applicability of the two models to the hydrodynamic lubrication analysis of parallel thrust bearings with surface textures, comparison between experiment and simulation results has been carried out on parallel thrust bearings in terms of cavitation zone morphology in a groove, friction coefficient, and bearing clearance. The results have shown that the observed cavitation morphology in steady state is more similar to the prediction from the JFO model than that from the Reynolds model.

Boiling heat transfer and pressure drop of R-134a and R-1234yf in minichannels for low mass fluxes

Analysis of various existing correlations of predicting local heat transfer and pressure drop in minichannels is performed. Heat transfer coefficient is compared to experimental results. Tests are conducted in a 6-channel tube. Channels are rectangular with 1.1 mm hydraulic diameter. Test parameters were varied in the following ranges: mass flux G = 20–100 kg m−2 s−1, heat flux q” = 2–15 kW m−2, and vapor quality 0–1.Two working fluids are used R-134a and R-1234yf.Contrary to previous studies, in this study convective boiling was identified as the dominant boiling regime. The effect of the critical heat flux on the heat transfer coefficient for low mass flux is also studied. The realized tests are compared to several correlations. Four different intervals of mass and heat fluxes are detected. A correlation of the heat transfer coefficient is proposed for these different intervals.

D202 中性子ラジオグラフィを用いたPEFC内における面方向水分布計測と流路-GDLにおけるガス流動解析(OS5 水素・燃料電池)

Visualization and measurement of water distributions in a polymer electrolyte fuel cell (PEFC) were carried out by means of neutron radiography for clarifying water effects on the performances. The two-dimensional water distributions were applied for a network model. It analyzes the gas-velocity distributions depending on the local pressure drop which is affected by the gas-flow rate and the water accumulation. Formation of the accumulated water in the channel was assumed based on measurement results of water distributions in through-plane direction. Furthermore, it was estimated that maximum water accumulation in the gas diffusion layer (GDL) and mechanism of water discharge from the GDL to the channel. From the analysis results, change of gas-velocity distributions depending on the water accumulation in the PEFC could be obtained.

HVAC 덕트 내에 설치된 광촉매코팅 모듈의 형상이 압력강하에 미치는 영향

The purpose of this study is to develop the shape of photocatalyst-coated module for improve the IAQ, which is installed at inside of ductwork and detachable. Including 3 column types(square, circle, diamond) and 2 fin types(diamond and square), totally 5 types are previously declared for numerical analysis and comparison, 5 types are. As the results of numerical analysis, almost the velocity varied at the range of from the module, except the Type A-3(diamond column type) which is affected to range and shows the biggest velocity differences. Among the 5 types, the diamond fin type(Type B-1) is analyzed as the most stable in velocity. And the results of local pressure drop show that the difference of pressure coefficient of Type B-1 is computed as 0.59, and that of Type A-3 is 2.44. Meanwhile, from the effect analysis of the number of module, the flow conflict happens and the pressure difference between before and after the module increases if there are over 3 modules inserted.

Field measurements of aerodynamic pressures in tunnels induced by high speed trains

The passage of high speed trains causes aerodynamic effects in tunnels, and considerable pressure transients are generated because of the restricted airspace within the tunnel. This leads to passenger discomfort, noise surrounding the tunnel, resistance to train movement, and possible damage to the train body and tunnel facilities. For the real assessment of the aerodynamic pressures in the tunnel induced by the passing trains of the high speed rail in Taiwan, a series of field measurements were performed near the portal and the shaft or the adit of the tunnel during normal operation. The measurements were conducted for several train speeds, including the maximum operation speed of the high speed rail in Taiwan, which is 300 km/h. Pressure sensors were deployed along the tunnel to investigate the propagation of pressure waves. The results show that the train nose entry/exit generated a compression wave propagating throughout the tunnel, resulting in a sharp increase in pressure. Conversely, the train tail entry/exit generated an expansion wave causing a pressure drop. The successive reflections of these pressure waves between both ends of the tunnel were observed. The pass-by of a train inside the tunnel also induced an immediate local pressure drop due to aerodynamic drag. Based on the measurement results, the spatial variation of the train-induced pressure peaks inside the tunnel is discussed. Furthermore, the relationship between the pressure peaks and the train speed is established, and the influence of the cross-sectional area of the tunnel is also presented.

Flow regime-based modeling of heat transfer and pressure drop in microchannel flow boiling

Local heat transfer coefficients and pressure drops during boiling of the dielectric liquid fluorinert FC-77 in parallel microchannels were experimentally investigated in recent work by the authors. Detailed visualizations of the corresponding two-phase flow regimes were performed as a function of a wide range of operational and geometric parameters. A new transition criterion was developed for the delineation of a regime where microscale effects become important to the boiling process and a conventional, macroscale treatment becomes inadequate. A comprehensive flow regime map was developed for a wide range of channel dimensions and experimental conditions, and consisted of four distinct regions – bubbly, slug, confined annular, and alternating churn/annular/wispy-annular flow regimes. In the present work, physics-based analyses of local heat transfer in each of the four regimes of the comprehensive map are formulated. Flow regime-based models for prediction of heat transfer coefficient in slug flow and annular/wispy-annular flow are developed and compared to the experimental data. Also, a regime-based prediction of pressure drop in microchannels is presented by computing the pressure drop during each flow regime that occurs along the microchannel length. The results of this study reveal the promise of flow regime-based modeling efforts for predicting heat transfer and pressure drop in microchannel boiling.

Effects of inclination angle on the steady flow and heat transfer of power-law fluids around a heated inclined square cylinder in a plane channel

The incompressible Poiseuille flow and heat transfer of a power-law fluid across an inclined long square cylinder confined in a two-dimensional channel is investigated numerically in the steady laminar flow regime. The effects of inclination angle, α, and power-law index, n, on the non-dimensional pumping power and the Nusselt number are studied for the conditions: 1 ⩽ Re ⩽ 40, 0 ⩽ α ⩽ 45, 0.4 ⩽ n ⩽ 1.8, Pr = 50 and blockage ratio β = 1/4. It is verified that the flow is steady in these ranges of conditions and the vortex shedding occurs only for n = 0.4 and α = 45 at Re ⩾ 33. In the steady flow regime the results show that the total drag coefficient on the cylinder increases with an increase in the power-law index and the maximum total lift coefficient occurs at α = 15. Moreover, a decrease in the power-law index and/or an increase in the inclination angle increase the Nusselt number and decrease the non-dimensional local pumping power. Simple correlations are also introduced for drag coefficient, local pressure drop and Nusselt number as functions of n, α and Re.

Pressure drop modeling and comparisons with experiments for single- and two-phase sodium flow

The thermal-hydraulic code TRACE is currently being extended at the Paul Scherrer Institute (PSI) for enabling the study of sodium-cooled fast reactor (SFR) core behavior during transients in which boiling is anticipated. An accurate prediction of pressure losses across fuel bundles – under both single- and two-phase sodium flow conditions – is necessary in this context. The present paper addresses the assessment, and implementation in TRACE, of appropriate friction factor models for round tubes and wire-wrapped fuel bundles, as well as local pressure drop models for grid spacers. Validity of the implemented correlations has been confirmed via the analysis of a range of experiments conducted earlier at the Joint Research Centre, Ispra. The measurements utilized are those of single- and two-phase pressure loss for sodium flow in tubes and 12-pin bundles, as a function of the inlet velocity under quasi steady-state conditions. The reported study thus represents an important further development step for the reliable simulation of two-phase sodium flow in TRACE.