Flow inside pipe, heat transfer

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Discussion Overview

The discussion revolves around the heat transfer in a heated pipe used to warm water from a lower temperature to a higher temperature. Participants explore calculations related to heat transfer, wall temperatures, and the effects of flow characteristics within the pipe. The scope includes theoretical and mathematical reasoning related to heat exchangers and fluid dynamics.

Discussion Character

  • Homework-related
  • Mathematical reasoning
  • Technical explanation
  • Debate/contested

Main Points Raised

  • Some participants propose using the heat transferred to the water (Q) to calculate wall temperatures at various points in the pipe, questioning if this approach is valid.
  • Participants discuss the implications of constant heat flux along the length of the pipe and its relation to the heat flux at the inner surface.
  • There is a challenge regarding the definition of heat flux, with some asserting it is the component perpendicular to the wall of the pipe.
  • Questions arise about the overall heat load of the pipe and the calculation of wall heat flux based on the inside surface area.
  • Some participants provide numerical values for the inside surface area and wall heat flux, while others seek clarification on these calculations.
  • The flow type is identified as turbulent, and a heat transfer coefficient is calculated, prompting further questions about temperature differences between the bulk fluid and the wall.
  • There is a query about whether the method of solving the problem is contingent on the assumption of constant heat flow across the pipe.

Areas of Agreement / Disagreement

Participants express differing views on the validity of using heat transfer calculations to determine wall temperatures, and there is no consensus on the implications of constant heat flux. The discussion remains unresolved regarding the best approach to these calculations.

Contextual Notes

Participants reference various assumptions, such as the medium temperature for specific heat capacity and the nature of the flow (laminar vs. turbulent), which may affect the calculations. There are also unresolved mathematical steps related to the heat transfer coefficient and temperature differences.

Kqwert
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Homework Statement


A 5m long heated pipe is used heat up water from 15°C to 65°C. Water flow through the pipe is 10liter/min. The heating gives a constant heat flow in all parts of the pipes surface. The inner and outer diameter of the pipe is 30 and 50 mm, respectively. Calculate the heat transferred to the water and the inner surface temperature of the pipe at the end (point where water leaves the pipe at temperature of 65oC)

Homework Equations

The Attempt at a Solution


When calculating the heat transferred to the water I used
Q = m,massflow*Cp*dT.

Cp was taken at the "medium" temperature of 40 celsius. My question is: Can you use the heat transferred to the water, i.e. Q, in calculating the wall temperatures at a given point in the pipe? I would then use Q = h*A,i*(Twall-65) and solve for Twall. h is calculated by using the Reynolds/Nusselt number,
 
Last edited:
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Kqwert said:

Homework Statement


A pipe with heating is used to heat up water from 5 celsius to 65 celsius. The volume flow through the pipe is 10L/min. The heated pipe gives a constant heat input over the length of the pipe. Find the effect of the heat exchanger and the temperature of the inner pipe wall at the exit.

Homework Equations

The Attempt at a Solution


When calculating the effect of the heat exchanger I used
Q = m,massflow*Cp*dT.

Cp was taken at the "medium" temperature of 40 celsius. My question is: Can you use the effect of the heat exchanger, i.e. Q, in calculating the wall temperatures at a given point in the pipe? I would then use Q = h*A,i*(Twall-65) and solve for Twall. h is calculated by using the Reynolds/Nusselt number,
What is the exact statement of the problem?
 
Chestermiller said:
What is the exact statement of the problem?
I have now edited my first post with the correct problem statement, as well as how I wanted to solve it, as I mixed it with another question while typing..
 
Kqwert said:
A 5m long heated pipe is used heat up water from 15°C to 65°C. Water flow through the pipe is 10liter/min. The heating gives a constant heat flow in all parts of the pipes surface. The inner and outer diameter of the pipe is 30 and 50 mm, respectively. Calculate the heat transferred to the water and the inner surface temperature of the pipe at the end (point where water leaves the pipe at temperature of 65oC)
If the heat flux is constant along the length of the pipe, what is the heat flux at the inner surface of the pipe?
 
Chestermiller said:
If the heat flux is constant along the length of the pipe, what is the heat flux at the inner surface of the pipe?
It is equal to the heat flux along the length of the pipe?
 
No. It is equal to the component of heat flux perpendicular to the wall of the pipe. What is the overall heat load of the pipe?
 
Chestermiller said:
No. It is equal to the component of heat flux perpendicular to the wall of the pipe. What is the overall heat load of the pipe?
That would be 34.6kW.
 
What is the inside surface area of the pipe? What is the wall heat flux at the inside surface of the pipe?
 
Chestermiller said:
What is the inside surface area of the pipe? What is the wall heat flux at the inside surface of the pipe?
The inside surface area of the pipe would be 0.47 m2. The wall heat flux is 34.6 kW / 0.47 m2 ?
 
  • #10
Kqwert said:
The inside surface area of the pipe would be 0.47 m2. The wall heat flux is 34.6 kW / 0.47 m2 ?
Can you please actually state that as a single number with units?
Is the flow (a) laminar or (b) turbulent?
 
  • #11
Sorry. 73.6 kW / m^2 I would say. The flow is turbulent.
 
  • #12
Kqwert said:
Sorry. 73.6 kW / m^2 I would say. The flow is turbulent.
Based on your heat transfer correlation, what is the heat transfer coefficient for the flow inside the pipe?
 
  • #13
Chestermiller said:
Based on your heat transfer correlation, what is the heat transfer coefficient for the flow inside the pipe?
It is 1548 W/m^2*K
 
  • #14
Based on the heat flux and this heat transfer coefficient, what is the temperature difference between the bulk fluid and the wall?
 
  • #15
That is 47 degrees, and Twall is then 112 celsius. Is this correct?
 
  • #16
Is this way of solving the problem only possible because the heating gives a constant heat flow across the length of the pipe? I.e. using the calculated total heat transferred in finding inner wall temperatures at different places across the pipe.
 

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