When flowing fluid passes through throttling there is a drop in

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In summary: well, in summary, because there is a pressure drop, i.e. there is an expansion, there is a drop in pressure and also there's a drop in temperature.
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ahmedbadr
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when flowing fluid passes through throttling there is a drop in pressure and also there's a drop in temperature why?can anyone help??
 
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  • #3


To help clarify, a pressure drop through a restriction does not do any work, and is normally estimated as an adiabatic process. As such, it is then an isenthalpic process (dH = 0).

If the gas is ideal (ie: PV = mRT) then the expansion (increase in V) will exactly counter the pressure drop (decrease in P) to give you a constant (ie: PV = constant for ideal gas). In the case of a real gas, this isn't always the case. Under typical circumstances, PV is slightly lower after the expansion, so the gas is very slightly cooler. However, there is often some range of values where PV will actually increase slightly after expansion, so the gas can actually increase in temperature. The best way to determine this temperature change is to determine the state downstream of your restriction from a gas properties database knowing the enthalpy upstream and the pressure downstream. These two (enthalpy and pressure) then define the state of the gas downstream and thus the temperature. The effects are generally very small and are due to the non-ideal nature of the gas.
 
  • #4


Also, for an Ideal Gas, the temperature remains constant.

CS
 
  • #5


well,u say there is expansion i don't know why there is expansion what causes this expansion??also i can get why pressure decreases in the constriction but according 2 bernoulli equation pressure will restore its value when fluid get out of constriction and back to flow in the same diameter before constriction but that doesn't happen??
 
  • #6


ahmedbadr said:
well,u say there is expansion i don't know why there is expansion what causes this expansion??also i can get why pressure decreases in the constriction but according 2 bernoulli equation pressure will restore its value when fluid get out of constriction and back to flow in the same diameter before constriction but that doesn't happen??

The pressure drop in a throttling device is caused by the increase in velocity. The velocity is increased due to the decrease in area of the device that the fluid is flowing into and then out of (e.g. throttling valve, porous plug, capillary tube). Take a look at the Bernoulli equation and the Continuity equation to see the relationship between pressure, area, and velocity. You'll see that energy is conserved.

Remember that a throttling process is defined as isenthalpic (h1 = h2).

Enthalpy is defined as:

h = u + Pv

Therefore, since pressure decreases, the specific volume must increase in order for h to remain constant (i.e. there is an expansion - assuming u is constant which is the case for an ideal gas). If u, the internal energy, is not constant, then you will see a temperature change like Q_Goest explained previously.

In order to expand (i.e. increase the specific volume), the fluid must do work. Since the process is adiabatic (and the KE and PE are negligible), the energy to do the work comes from the internal energy of the fluid. This typically results in a decrease in temperature (in real gases). However, depending on the inversion point, the temperature may actually increase. This is typically referred to as the Joule-Thompson effect, and the J-T coefficient determines if the temperature goes up or down. If the J-T coefficient is negative, the temperature will increase. If it is positive, the temperature will decrease. If it is zero, the temperature remains constant (again this is only for an ideal gas).

Hope this helps.

CS
 

1. What causes a drop in pressure when flowing fluid passes through a throttling device?

When a fluid passes through a throttling device, the flow area decreases, leading to an increase in velocity. According to Bernoulli's principle, as the velocity of a fluid increases, the pressure decreases. Therefore, the drop in pressure is caused by the increase in fluid velocity.

2. How does the type of fluid affect the magnitude of pressure drop in a throttling process?

The type of fluid affects the magnitude of pressure drop in a throttling process because it determines the fluid's viscosity and compressibility. Highly viscous fluids, such as honey, will experience a larger pressure drop compared to less viscous fluids, such as water. Additionally, gases, which are more compressible than liquids, will experience a larger pressure drop in a throttling process.

3. Is there a relationship between the rate of flow and the pressure drop in a throttling process?

Yes, there is a direct relationship between the rate of flow and the pressure drop in a throttling process. As the flow rate increases, the pressure drop also increases. This is because a higher flow rate means a larger change in velocity, according to Bernoulli's principle, resulting in a larger pressure drop.

4. How does the shape and size of a throttling device affect the pressure drop in a fluid flow?

The shape and size of a throttling device can significantly influence the pressure drop in a fluid flow. A smaller and more streamlined throttling device will create a higher velocity of fluid flow, resulting in a larger pressure drop. Additionally, the shape and size of the device can also affect the flow pattern of the fluid, further impacting the pressure drop.

5. What are the practical applications of understanding pressure drop in a throttling process?

Understanding pressure drop in a throttling process is crucial in various practical applications, such as designing pipelines, valves, and flow control systems. It also plays a significant role in industries that involve the transportation of fluids, such as oil and gas, chemical, and food processing. Additionally, knowledge of pressure drop can help identify and troubleshoot issues in fluid flow systems, ensuring efficient and safe operation.

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