An 8 m/s particle collides onto a falling droplet (0.8 m/s) as shown in Fig. 1. I need to write the energy balance equation. Kinetic energy of droplet and particle together before impact should be equal to the particle KE after impact plus energy loss (surface energies and KE of liquid after impact are small)
I can assume two sources for energy loss during this process: viscous dissipation and drag force applied to the particle.
My question is that, whether the energy loss due to the drag force is already calculated within the formula presented for viscous dissipation in Fig. 2 or I should calculate the drag loss separately?
Dear Sayed,
Energy loss due to to many factors:
-Viscous dissipation,
-Roughness,
-gravity,
-work of fluid against wall, etc.
Best regards!
The drag force of a particle embedded in a fluid is the force due to dissipation. Both things, dissipation force and drag force, are the same, so you should calculate it only once.
Dear Sayed,
Drag and viscous force means the same thing, just that the former is a more layman term, while the latter is a more technical term. They can be used interchangeably.
With my best regards
Prof. Bachir ACHOUR
Thank you. Therefore, if I insert the formula shown in the picture above in a Custom Field Function in Fluent to calculate the energy dissipation, and I also extract the drag force directly from the Fluent and then calculate the energy waste due to drag force, do you mean these two values should be the same (~equal)?
Thanks
Dear Sayed
However small it may be, the process clearly includes changes in the gravitational potential energy of both particle moving up and droplet moving down.
As for viscous dissipation, it corresponds to energy lost by the fluid as heat, actually transferred to the fluid surroundings, that is, the ambient air and the particle itself. The process is also accompanied by an increase in the fluid internal energy, responsible for an increase in its temperature.
Hi Amirhosein
For a finite Control Volume (e. g., a given pipe length) through which a fluid flows, the energy dissipation equals the viscous or friction head loss. This a term present in the, so called Mechanical Energy Equation (also referred to by other names, e. g., Macroscopic Energy Balance, Modified Bernoulli Equation etc.). Usually, this equation is written with terms having units of energy per unit weight of fluid, that is the height of a fluid column. The equation is restricted to steady flows of incompressible fluids in control volumes having one entrance and one exit ports and uses mean values of pressure, velocity and elevation at these ports.
For a more general differential analysis of fluid flow (unsteady, compressible), the viscous dissipation term is – [div (τ•v)], where τ is the stress tensor and v is the velocity vector, with units of power per unit volume of fluid. See, for instance, Transport Phenomena - Bird, Stewart and Lightfoot (1960, 2002).
The condition of laminar flow (your question) is in fact the simplest case.
https://www.researchgate.net/post/Energy_loss_from_flow_field
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