The Critical Velocity With Minimum Momentum Change in Pushing a Droplet Through a Constriction Channel

2016 ◽  
Author(s):  
Zhifeng Zhang ◽  
Xinpeng Zhao ◽  
Jie Xu ◽  
Xiaolin Chen
Keyword(s):  
Surface Tension ◽  
Critical Velocity ◽  
Pressure Loss ◽  
Biological Process ◽  
Dynamic Pressure ◽  
Theoretical Work ◽  
Force Balance ◽  
Channel Inlet ◽  
Average Surface ◽  
Momentum Change

Models of squeezing a droplet through a constrictions have wide applications in nature and engineering. In the present research, we found the minimum impulse required (momentum change) to squeeze a droplet through at different velocities. Our theoretical work result in an analytical expression of the critical velocity with minimum impulse. At this expression, we found a force balance between the average surface tension and the dynamic pressure loss at the channel inlet and outlet. Finally, we also compared the analytical results with numerical simulations. These results are important to understand some biological process and design of microscale filters.

Aerodynamic Similarity Principles and Scaling Laws for Windmilling Fans

2018 ◽  
Author(s):  
Dilip Prasad
Keyword(s):  
Rotational Speed ◽  
Pressure Loss ◽  
Scaling Laws ◽  
Dynamic Pressure ◽  
Stagnation Pressure ◽  
Design Parameters ◽  
Similarity Parameter ◽  
Wide Range ◽  
Simulation Results ◽  
Good Agreement

Windmilling requirements for aircraft engines often define propulsion and airframe design parameters. The present study is focused is on two key quantities of interest during windmill operation: fan rotational speed and stage losses. A model for the rotor exit flow is developed, that serves to bring out a similarity parameter for the fan rotational speed. Furthermore, the model shows that the spanwise flow profiles are independent of the throughflow, being determined solely by the configuration geometry. Interrogation of previous numerical simulations verifies the self-similar nature of the flow. The analysis also demonstrates that the vane inlet dynamic pressure is the appropriate scale for the stagnation pressure loss across the rotor and splitter. Examination of the simulation results for the stator reveals that the flow blockage resulting from the severely negative incidence that occurs at windmill remains constant across a wide range of mass flow rates. For a given throughflow rate, the velocity scale is then shown to be that associated with the unblocked vane exit area, leading naturally to the definition of a dynamic pressure scale for the stator stagnation pressure loss. The proposed scaling procedures for the component losses are applied to the flow configuration of Prasad and Lord (2010). Comparison of simulation results for the rotor-splitter and stator losses determined using these procedures indicates very good agreement. Analogous to the loss scaling, a procedure based on the fan speed similarity parameter is developed to determine the windmill rotational speed and is also found to be in good agreement with engine data. Thus, despite their simplicity, the methods developed here possess sufficient fidelity to be employed in design prediction models for aircraft propulsion systems.

Aerodynamic Similarity Principles and Scaling Laws for Windmilling Fans

2018 ◽  
Vol 140 (12) ◽  
Author(s):  
Dilip Prasad
Keyword(s):  
Rotational Speed ◽  
Pressure Loss ◽  
Scaling Laws ◽  
Dynamic Pressure ◽  
Stagnation Pressure ◽  
Design Parameters ◽  
Similarity Parameter ◽  
Wide Range ◽  
Simulation Results ◽  
Good Agreement

Windmilling requirements for aircraft engines often define propulsion and airframe design parameters. The present study is focused on two key quantities of interest during windmill operation: fan rotational speed and stage losses. A model for the rotor exit flow is developed that serves to bring out a similarity parameter for the fan rotational speed. Furthermore, the model shows that the spanwise flow profiles are independent of the throughflow, being determined solely by the configuration geometry. Interrogation of previous numerical simulations verifies the self-similar nature of the flow. The analysis also demonstrates that the vane inlet dynamic pressure is the appropriate scale for the stagnation pressure loss across the rotor and splitter. Examination of the simulation results for the stator reveals that the flow blockage resulting from the severely negative incidence that occurs at windmill remains constant across a wide range of mass flow rates. For a given throughflow rate, the velocity scale is then shown to be that associated with the unblocked vane exit area, leading naturally to the definition of a dynamic pressure scale for the stator stagnation pressure loss. The proposed scaling procedures for the component losses are applied to the flow configuration of Prasad and Lord (2010). Comparison of simulation results for the rotor-splitter and stator losses determined using these procedures indicates very good agreement. Analogous to the loss scaling, a procedure based on the fan speed similarity parameter is developed to determine the windmill rotational speed and is also found to be in good agreement with engine data. Thus, despite their simplicity, the methods developed here possess sufficient fidelity to be employed in design prediction models for aircraft propulsion systems.

Transient Pressure Loss in Compressor Station Piping Systems

2010 ◽  
Author(s):  
Klaus Brun ◽  
Marybeth Nored ◽  
Dennis Tweten ◽  
Rainer Kurz
Keyword(s):  
Pressure Loss ◽  
Dynamic Pressure ◽  
Piping System ◽  
Flow Profile ◽  
Fluid Models ◽  
Reciprocating Compressor ◽  
Fluid Structure Interactions ◽  
Fluid Structure ◽  
Pressure Losses ◽  
Piping Systems

“Dynamic pressure loss” is often used to describe the added loss associated with the time varying components of an unsteady flow through a piping system in centrifugal and reciprocating compressor stations. Conventionally, dynamic pressure losses are determined by assuming a periodically pulsating 1-D flow profile and calculating the transient pipe friction losses by multiplying a friction factor by the average flow dynamic pressure component. In reality, the dynamic pressure loss is more complex and is not a single component but consists of several different physical effects, which are affected by the piping arrangement, structural supports, piping diameter, and the level of unsteadiness in the flow stream. The pressure losses due to fluid-structure interactions represent one of these physical loss mechanisms and are presently the most misrepresented loss term. The dynamic pressure losses, dominated at times by the fluid-structure interactions, have not been previously quantified for transient flows in compressor piping systems. A number of experiments were performed by SwRI utilizing an instrumented piping system in a compressor closed loop facility to determine this loss component. Steady and dynamic pressure transducers and on-pipe accelerometers were utilized to study the dynamic pressure loss. This paper describes findings from reciprocating compressor experiments and the various fluid modeling studies undertaken for the same piping system. The objective of the research was to quantitatively assess the individual pressure loss components which contribute to dynamic pressure (non-steady) loss based on their physical basis as described by the momentum equation. Results from these experiments were compared to steady state and dynamic pressure loss predictions from 1-D and 3-D fluid models (utilizing both steady and transient flow conditions to quantify the associated loss terms). Comparisons between the fluid model predictions and experiments revealed that pressure losses associated with the piping fluid-structure interactions can be significant and may be unaccounted for by advanced 3-D fluid models. These fluid-to-structure losses should not be ignored when predicting dynamic pressure loss. The results also indicated the ability of an advanced 1-D Navier Stokes solution at predicting inertial momentum losses. Correspondingly, the three-dimensional fluid models were able to capture boundary layer losses affected by 3-D geometries.

A Generalized Model for Dynamic Contact Angle

2017 ◽  
Author(s):  
Joseph J. Thalakkottor ◽  
Kamran Mohseni
Keyword(s):  
Surface Tension ◽  
Contact Angle ◽  
Contact Angle Hysteresis ◽  
Dynamic Contact Angle ◽  
Dynamic Contact ◽  
Force Balance ◽  
Surface Tension Gradient ◽  
Static Case ◽  
Dynamic Case ◽  
Microscopic Dynamic

Contact angle is an important parameter that characterizes the degree of wetting of a material. While for a static case, estimation and measurement of contact angle has been well established, same can not be said for the dynamic case. There is still a lack of understanding and consensus as to the fundamental factors governing the microscopic dynamic contact angle. With the aim of understanding the physics and identifying the parameters that govern the actual or microscopic dynamic contact angle, we derive a model based on first principles, by performing a force balance around the region containing the contact line. It is found that in addition to the surface tension, the microscopic dynamic contact angle is also a function of surface tension gradient and the jump in normal stress across the interface. In addition to having a significant contribution in determining the microscopic dynamic contact angle, surface tension gradient is also a key cause for contact angle hysteresis.

scholarly journals Numerical simulation of the regime and geometric characteristics influence on the pressure loss of a low-flow aerothermopressor

2019 ◽  
Vol 55 (2) ◽  
pp. 66-76
Author(s):  
H. Kobalava ◽  
D. Konovalov
Keyword(s):  
Pressure Loss ◽  
Dynamic Pressure ◽  
Air Flow ◽  
Low Flow ◽  
Cfd Modeling ◽  
Maximum Pressure ◽  
Two Phase ◽  
Working Chamber ◽  
Flow Part

In this paper, a study of gasdynamic processes that occur in a low-flow aerothermopressor has been done. The aerothermopressor is a two-phase jet apparatus for contact cooling, in which, due to the removal of heat from the air flow, the air pressure is increased (thermogasdynamic compression) and its cooling is taken place. Highly effective operation of the aerothermopressor is influenced by primarily the flow part design and the water injected method in the apparatus. Constructive factors that influence energy costs to overcome friction losses and local resistances on the convergent-divergent sections of the aerothermopressor are exerted a significant impact on the working processes in the apparatus. In this paper, a study of a number of typical low-flow aerothermopressor models has been conducted by using computer CFD modeling. Determination of the main parameters of the air flow (total pressure, dynamic pressure, velocity, temperature, etc.) has been carried out for a number of taper angles of a confuser a and a diffuser b, as well as for a number of values of the relative air velocity in the working chamber M = 0.4-0.8. Comparison of the obtained data with experimental data has been carried out. The deviation of the calculated values of local resistances coefficients in the confuser and in the diffuser from those obtained by computer CFD modeling does not exceed 7–10%. The recommended angles were determined: confuser convergent angle – 30° and diffuser divergent angle – 6°, corresponding to the minimum pressure loss is 1.0 – 9.5 %, and therefore also to the maximum pressure increase as a result of the thermogasdynamic compression that occurs during injection and evaporation of liquid in the working chamber. Thus, analytical dependences are obtained for determining the local resistance coefficients for the confuser (nozzle) and the diffuser, which can be recommended to use in the design methodology for low-flow aerothermopressors.

A Scale Analysis Based Theoretical Force Balance Model for Critical Heat Flux (CHF) During Saturated Flow Boiling in Microchannels and Minichannels

2010 ◽  
Vol 132 (8) ◽  
Author(s):  
Satish G. Kandlikar
Keyword(s):  
Heat Transfer ◽  
Surface Tension ◽  
Heat Flux ◽  
Critical Heat Flux ◽  
Flow Boiling ◽  
Narrow Channel ◽  
Force Balance ◽  
Balance Model ◽  
Scale Analysis ◽  
Error Band

Accurate prediction of critical heat flux (CHF) in microchannels and minichannels is of great interest in estimating the safe operational limits of cooling systems employing flow boiling. Scale analysis is applied to identify the relevant forces leading to the CHF condition. Using these forces, a local parameter model is developed to predict the flow boiling CHF. The theoretical model is an extension of an earlier pool boiling CHF model and incorporates force balance among the evaporation momentum, surface tension, inertia, and viscous forces. Weber number, capillary number, and a new nondimensional group introduced earlier by Kandlikar (2004, “Heat Transfer Mechanisms During Flow Boiling in Microchannels,” ASME J. Heat Transfer, 126, pp. 8–16), K2, representing the ratio of evaporation momentum to surface tension forces, emerged as main groups in quantifying the narrow channel effects on CHF. The constants in the model were calculated from the available experimental data. The mean error with ten data sets is 19.7% with 76% data falling within ±30% error band and 93% within ±50% error band. The length to diameter ratio emerged as a parameter indicating a stepwise regime change. The success of the model indicates that flow boiling CHF can be modeled as a local phenomenon and the scale analysis is able to reveal important information regarding fundamental mechanisms leading to the CHF condition.

Transient Pressure Loss in Compressor Station Piping Systems

2011 ◽  
Vol 133 (8) ◽  
Author(s):  
Klaus Brun ◽  
Marybeth Nored ◽  
Dennis Tweten ◽  
Rainer Kurz
Keyword(s):  
Pressure Loss ◽  
Dynamic Pressure ◽  
Piping System ◽  
Flow Profile ◽  
Fluid Models ◽  
Reciprocating Compressor ◽  
Fluid Structure Interactions ◽  
Fluid Structure ◽  
Pressure Losses ◽  
Piping Systems

“Dynamic pressure loss” is often used to describe the added loss associated with the time varying components of an unsteady flow through a piping system in centrifugal and reciprocating compressor stations. Conventionally, dynamic pressure losses are determined by assuming a periodically pulsating 1D flow profile and calculating the transient pipe friction losses by multiplying a friction factor by the average flow dynamic pressure component. In reality, the dynamic pressure loss is more complex and is not a single component but consists of several different physical effects, which are affected by the piping arrangement, structural supports, piping diameter, and the level of unsteadiness in the flow stream. The pressure losses due to fluid-structure interactions represent one of these physical loss mechanisms and are presently the most misrepresented loss term. The dynamic pressure losses, dominated at times by the fluid-structure interactions, have not been previously quantified for transient flows in compressor piping systems. A number of experiments were performed by Southwest Research Institute (SwRI) utilizing an instrumented piping system in a compressor closed-loop facility to determine this loss component. Steady and dynamic pressure transducers and on-pipe accelerometers were utilized to study the dynamic pressure loss. This paper describes the findings from reciprocating compressor experiments and the various fluid modeling studies undertaken for the same piping system. The objective of the research was to quantitatively assess the individual pressure loss components, which contribute to dynamic pressure (nonsteady) loss based on their physical basis as described by the momentum equation. Results from these experiments were compared with steady-state and dynamic pressure loss predictions from 1D and 3D fluid models (utilizing both steady and transient flow conditions to quantify the associated loss terms). Comparisons between the fluid model predictions and experiments revealed that pressure losses associated with the piping fluid-structure interactions can be significant and may be unaccounted for by advanced 3D fluid models. These fluid-to-structure losses should not be ignored when predicting dynamic pressure loss. The results also indicated the ability of an advanced 1D Navier–Stokes solution at predicting inertial momentum losses. Correspondingly, the three-dimensional fluid models were able to capture boundary layer losses affected by 3D geometries.

Study on Pipeline Self-Flowing Transportation of Cemented Tailing Fill Slurry Based on Fluent Software

2013 ◽  
Vol 734-737 ◽  
pp. 833-837
Author(s):  
Gen Bo Yu ◽  
Peng Yang ◽  
Zan Cheng Chen ◽  
Rui Ying Men
Keyword(s):  
Numerical Simulation ◽  
Critical Velocity ◽  
Flow Rate ◽  
Pressure Loss ◽  
Filling Material ◽  
Simulation Research ◽  
Fluent Software ◽  
Slurry Flow Rate ◽  
Pipeline Design ◽  
Calculation Analysis

The related calculation analysis and numerical simulation is the basis of filling pipeline design and installation. Based on characteristics testing of the filling material, the related calculation was performed on the filling slurry gravity transport by pipelines. Chosen pipeline whose diameter was 75 mm, the natural flowing speed can reach 2.6 m/s on condition of the highest concentration for 65%, which is greater than the critical velocity and can meet the needs of the production of the mine. Numerical simulation research shows that the flowing speed of system was relatively ideal, slurry flow rate of bend place has great change while pressure uniformity in the rest parts, and the pressure loss is 0.8 MPa. Therefore, it is capable to achieve gravity flow transporting.

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