Nanofluids are fluids containing nanometer (~1-100*10-9m) sized particles. It is engineered colloidal suspension of solid state nano-particles in a base fluid. Experiments suggest that the colloidal mixture has a higher heat transfer co-efficient compared to the base fluid. This is proving extremely useful as electronic devices get smaller and smaller with heat rejection becoming more critical. For example, inadequate heat transfer is the reason behind mobiles getting hot after extended usage and reduced performance of the processor.
So you may ask, isn’t there any other way to increase heat transfer. Why develop colloidal solutions for increasing heat transfer?
Why contemporary methods of heat transfer are not adequate
Let us simplify the problem of heat transfer (conduction and convection) a little by assuming 1-D heat transfer through a certain medium (be it solid or fluid). The most preliminary equation determining heat transfer between two infinite heat sources is,
where H: Heat transfer, A: surface area , ∇T : temperature difference between two ends of the medium and k being material constant.
So far, engineers have struggled to improve heat transfer rates. They expanded the surface area by providing fins in the case of conduction and convection heat transfer. They also tried to increase the flow rate of fluid over the surface to improve convection heat transfer. However, we have now reached a limit. Further research in that direction is mediocre at best (compared to the significant increase in heat transfer by using nanofluids).
Short answer? Because micro-sized particles tend to clog the extremely small ducts in certain electronic applications. Micro-sized particles also affect the performance and life span of the pump. Consequently, as particle sizes increase the difficulty of maintaining the colloidal solution also increases. It is extremely difficult to maintain a colloidal solution of micro particles.
Long answer? Consider a metallic sphere of radius R, volume V, and surface area A. Let us melt it and make small spheres of half the radius (R/2). We will end up with 8 such spheres and simple maths will lead us to the conclusion that the combined surface area of all the smaller spheres is twice the original one. Similarly, if we melt the original sphere into small spheres of one-third the radius (R/3) the combined surface area will be 3 times the original sphere.
Suppose we can melt the original sphere and make nano-meter sized spheres from it. We would end up with a surface area 10^9 times the surface area of the original sphere. It is clear that the higher surface area leads to a higher heat transfer rate. But the basic problem is, how will you utilize this surface area? This would have been a logical solution if heat was generated at the core of each sphere. However, heat travels from one sphere to another and the contact between two spheres is a point contact. So the increase in surface area doesn’t solve the problem.
As you might have guessed, we can let these nano-meter sized particles create a colloidal solution in liquid. That way, half of the 10^9 surface area can be used to receive the heat and other half to reject the heat to the adjacent particles. The problem of point contact is resolved as liquid is in constant touch with the sphere and we can utilize all the surface area the sphere has to offer. The only drawback of this scheme is that liquid has comparatively poor thermal conductivity and hence, it reduces the overall heat transfer rate. The 0.5*10^9 improvement we hoped for, reduces and depends largely on the the thermal conductivity of the base fluid used. Still, the combined thermal conductivity is way higher than whatever the liquid can offer.
Here is a chart depicting thermal conductivity of certain liquid and solid elements for comparison purposes.
|Substance||Thermal conductivity(at 00C)(W/m.oC)|
|Lubricating oil(SAE 50)||0.0146|
As you may observe, liquid has significantly lower thermal conductivity compared to metal. Nanofluids which behave like a fluid, help in achieving considerably higher rates of heat transfer. They offer higher thermal conductivity while still maintaining the properties of the fluid.