dear Nazaruddin Sinag, if you ar trying of reproduce an isothermal condition in the pipe wall the best way in my opinion is put the whole pipe surrounded by a fluid suffering a phase change. for example you can built like a tube in tube heat exchanger and in the space between pipes you can have boiling water or condensing steam. on the other hand for keep a constant heat flow you can use electrical heating because for the same level of voltage supply always you will have the same heat flow.

Dear Nazaruddin,

For constant heat flux condition, use a heating tape with a variable input current drive.

Dear Ruben Borrajo and Syed Mughees Ali,

Thanks for your suggestion, especially for using phase change fluid to produce isothermal condition. However, how do you think about using electrical heating for making a constant heat flux condition in the pipe wall, particularly if the flow is still developing. In this case, the heat transfer coefficient of the fluid is not constant. So, there will be a nonuniformity of the heat flux in the electrical heater. Please correct me if I am wrong.

Thank you

Dear İbrahim Halil Yılmaz

The inlet temperature of the fluid (water) is 30 oC and the outlet temperature up to 90 oC. The pipe length is 80 cm and the pipe diameter is 2 in.

In order to obtain a constant power flux you should take into consideration the resistor resistivity changes versus temperature not only maintain a constant current. If you want to maintain a constant wall temperature the best way is to build up a control loop since depending on the internal flow the convection will change and the wallo temperature accordingly. The loop has to be faster than the flow changes and have an integral part in order to lead to zero error but in your measurements you should take into consideration the thermal inertia and make them after the transient period when the steady state has been reached.

Dear all,

How do you think about using electrical heating for making a constant heat flux condition in the pipe wall, particularly if the flow is still developing? In this case, the heat transfer coefficient of the fluid is not constant. So, there will be a nonuniformity of the heat flux in the electrical heater. Please advice me.

Thank you...

You mix power flux and temperature. There are 2 different situations : constant power flux and constant temperature. In the 1st case temperature is variable in the second the dissipated power is changing. If you read what I wrote you know what to do. Think it over again.

Dear Nick Hamburger

Thanks for your response. I realized this condition, but the problem is: how to make a constant temperature or constant heat flux condition in the developing flow region? I think this case is different with the fully developed flow region....

Thanks Ibrahim. I am happy reading your explanation.

Please imagine a small HEX. Most of the region is in the developing region, where both the mass and heat tranfer change with the distance from the inlet region.....

The only way is to measure the flux in the direction you want to have it constant and build up a control loop in order to maintain it constant which ever will be the temperature difference due to the convection variations. As a hint the temperature difference across a wall is proportional to the flux / wall thermal conductivity . So that you can have a measure of the temperature difference and adjust your thermal source compensating its variations. This way you are independent of the convective effects. In the case of a constant temperature you may use same loop but limiting to one temperature measurement instead of a difference. As temperature sensors you may use for instance Pt1000 with a quite large TCR and build a bridge to increase the overall sensitivity.

If you still have difficulties to understand let me know and I shall send you an explanatory sketch.

Thanks Nick,

Please send me the sketch.

Regards

Nazar

Hi Nazaruddin Sinaga,

Here you have the principle explained. I suggest 4 sensor pairs but it depends on the pipe wall conductivity. If it is a metallic wall then only one pair is enough.

I hope considering all your skills that everything is clear if not ask again about the details you did not understand. You can use the bridge output as feedback for the loop and you can control the loop with a PWM since thermal process are slow speed.

Good luck

Thanks Nick. I will learn it....

Dear Researchers,

Have a nice weekend with your family....

Having a constant flow is simpler than a constant temperature.

The constant flow can be obtained with a constant heating power: an ohmic wire travaersed by a constant current intensity, an induction plate, or a flame with constant gas flow (with stable combustion).

Constant temperature is a closed-loop control problem. Temperature sensors, an electronic comparator, a control device for the heating tool are required, and PID control.

You can find the details in the automatic and regulation books.

When one considers convection, constant heat flux and constant wall temperature comes into picture. Constant heat flux is obtained by surrounding the tube by a wire giving low voltage and high current to avoid shock. This is easier to do and hence constant heat flux is preferred. The other is constant wall temperature where the tube is surrounded by steam jacket.