Dear Dr. Mohamed

Thank you for this detailed explanation that contributes to the approximation of the answer to the original question.

Kindly regards

Dear Dr. Hayder Jaffal,

There are about fourteen enhancement techniques introduced by Bregles et al. 1983 about improvement Heat transfer especially in heat exchangers. one of the most important method is additives such (Nanoparticles) for liquids and classified as a passive method which do no need any external power like in active methods. my regards

you can take a look at this review paper

Dear Mohamed Saad

Thanks a lot to answer my question. It is known that the rate of heat transfer can be actively enhanced by extra power, but I read paper classified the nanofluid as active method which made me in confused for this classification.

Kindly regards

Read this paper:

https://irjet.net/archives/V3/i9/IRJET-V3I9292.pdf

Adding nano particles to the base fluid is contributed to the Passive technique. Other passive techniques involve using helical coil, adding extended surfaces, etc.

All Additives (gas into liquides; air bubles in liquids or solid suspension in liquids "nanofluids") are considered as passive techniques, cuz they don't need any external input or power contrary to active techniques.

Greetings

I would like to thank all RG Colleagues for the useful answers.