In fact the counter productive happen at nano fluid volume concentration of 0.5%,1% and 2%

Dear Mahmoud

Addition of nanoparticales to base fluid improves heat transfer in the heat exchangerس due to increased thermal conductivity of the new fluid. On the other hand, it has a negative effect is the increase of pressure gradient resulting from increased viscosity of the new fluid. Using the performance factor [P.F=(Nu/Nub)/(f/fb)^0.333] can determine the benefit of the addition of nanoparticales to the base fluid. The more the performance factor than one, the addition of nanoparticales would be more useful and if the coefficient less from one the addition is useless.

Dear Mahmoud

Yes, it is possible. The addition of nanoparticles in the base-fluid undeniably increase the effective thermal conductivity. However, the changes in other effective properties by the addition of nanoparticles (such as specific heat, density and viscosity) may have an adverse affect on the nanofluid application. Therefore, the combined impact of all properties must be considered. Other than that, the stability of nanofluid plays an important role and there is a huge possibility that the nanofluid may lose the stability at high operational temperatures in a heat exchanger.

You may refer to the following publications for more information

Article Experimental investigation of natural convection heat transf...

Chapter Stability of Nanofluids

Dear Mahmoud,

I agree with Hayder's answer

In my case, the pressure gradients became higher for higher concentration of nanofluids even though the effective thermal conductivity was slightly higher.

I thought it is also affected by what kind of system you used for your nanofluids.

Yes, nanofluids (Al2O3, CuO, zinc oxide, titanium oxide, carbon nano tubes etc.) by 0.5% to 3% of volume concentration in combination with the base fluid (water etc.) surely increases heat transfer in heat exchangers. The reason is nano fluids have nano particles of size varying from 20 nano particles to 50 nano particles which has higher thermal conductivity, high heat capacities, good oxidation, larger surface area to volume ratio etc

Thanks for all of you about the exciting help

Dear Mahmoud,

In addition to increased pumping power requirement (which is an application-based challenge), it is important to note that the theoretically proven and experimentally shown improvements in thermophysical properties may not be totally sustained for unstable nanofluids over time (preparation- and storage-based challenge). Once the colloidal stability is deteriorated, the effective nanoparticle fraction decreases due to sedimentation, and relative improvements in thermophysical properties may become different than that predicted by theory.

You may want to check the article by [Mahbubul et al. 2019, Powder Technology 345, 668-675], which discusses nanofluid performance in relation to colloidal stability.

This is a tricky question, proven by the conflicting findings often reported in the literature.

Even by assuming a completely stable and homogeneous nanoparticle distribution (which is almost never the case), and even when we ignore any detrimental effects on viscosity or heat capacity, the effect of nanoparticles on heat transfer largely depends on operational and flow parameters, such as Re and Ra for forced and natural convection.

In my humble opinion, any marginal augmentation of heat transfer that (may) take place is completely overshadowed by the fact that the dispersion and chemical stability of nanofluids (which are basically nanocolloids) at high operation temperatures is very questionable and unproven. The higher your temperature, the higher the rates of aggregation and sedimentation become, and the higher the likelihood of chemical instability issues with surfactants or the nanoparticles themselves.

I do believe that nanofluids should be utilized in applications that build on their exceptional optical, electronic, magnetic, and catalytic properties. Using them in realistic heat transfer applications by merit of their thermal properties is unreliable, at best.

In my opinion nanofluids increase the heat transfer. Increasing heat transfer with nanofluids depends on many factors. The most important factor among these factors is stability. If you cannot provide stability, nanofluids will have a negative effect on heat transfer in heat exchangers.[ http://dergipark.gov.tr/download/article-file/509103 ]

As everyone has pointed out, the nanofluid has to be engineered in such way that heat transfer enhancements overcome viscosity increase, and at the same time the nanofluid has been kept stable. This paper of our group (attached) describes a diamond nanofluid that is ultimately stable and good TC enhancement with minimum penalty in viscosity is attained.