Experimental Study On Heat Transfer Of Nanofluids In A Vertical Tube Under Supercritical Pressure

The high temperatures and high heat flux in the engines of supersonic aircrafts make it a necessity for efficient cooling systems. The regenerative cooling system is thought to be an effective and practical solution to better thermal management. Engine fuels such as kerosene are commonly used as regenerative coolants. As nanofluids have extraordinary heat transfer characteristics, such as high thermal conductivity, we proposed nanofluid fuel as a new type of regenerative coolants. Experiments were done to study the heat transfer characteristics of nanofluid fuels flowing in a vertical tube under supercritical pressure.Two-step method was applied to prepare Al2O3-kerosene and Fe3O4-kerosne nanofluids. The effects on heat transfer of pressure (2.5～4.5MPa), mass flow rate (2.0～3.5g/s), heat flux (150～300kW/m2) and nanoparticle content (0.02～0.1wt%) were studied. Results are as follows:(1) Under supercritical pressure, there are different heat transfer mechanisms of nanofluids flowing inside a vertical tube in the inlet, middle and outlet regions. At the inlet region, the heat transfer coefficient is determined by the thickness and the turbulences in the thermal boundary layer. At the outlet region, the supercritical nanofluids near the tube wall have low density, low viscosity and low thermal conductivity. And the near-wall nanofluid forms a film and prevents the efficient heat transfer of the bulk flow to the tube wall. The middle region is a transition between these two mechanisms.(2) Increasing the flow rate or working pressure could enhance the heat transfer performances of nanofluids flowing in a vertical under supercritical pressure. As flow rate increases, the Reynolds number will also increase and heat transfer is enhanced. The nanofluid with high flow rate will also wash away the nanoparticles deposited on the tubes, so heat resistance will decrease. Additionally, higher pressure will restrain the film-like flow near the tube wall, so the heat transfer performances will be better.(3) The effects of heat flux on heat transfer are complicated. Higher heat flux will make the temperature difference between fluid and tube wall larger, so the mixed convection will be initiated to enhance heat transfer. Meanwhile, the film-like flow near the tube wall will be initiated by higher tube wall temperature, so heat transfer coefficients will be deteriorated.(4) The addition of nanoparticles deteriorates the heat transfer performances of Al2O3-kerosene and Fe3O4-kerosne nanofluids in a vertical tube under supercritical pressure. As the particle content increases, the opportunities of crash between particles and tube wall or between different particles will also increase. Thus, the inner tube wall surface becomes smoother and the heat transfer performance is deteriorated.