A study of drag reduction and convective heat transfer reduction in turbulent flow through pipe

Addition of a small quantity of certain long chain polymers or micelle forming surfactant additives to a fluid will cause the frictional drag and convective heat transfer to be reduced in turbulent pipe flow. Reynolds number has been proven to be inadequate for characterizing the flow of such drag reducing solutions in different diameter pipes.;The reduction in drag and convective heat transfer occurs due to dampening of the velocity fluctuations in the turbulent boundary layer. Velocity fluctuations close to the wall are dependent upon shear stress, and are independent of the pipe diameter. Consequently, reduction in drag and heat transfer, calculated at a constant shear stress, should also be independent of the pipe diameter. The above hypothesis was successfully tested on polymer-oil and surfactant-water systems.;Dampening of velocity fluctuations results in thickening of the laminar sublayer and can also be equated to a reduction in Prandtl's mixing length. Velocity-shear stress data generated by flow experiments in a laboratory scale pipe along with viscosity measurements enable determination of the modified mixing length constant and the thickness of the laminar sublayer for a given drag reducing fluid, which can be subsequently used for predicting flow and heat transfer in any diameter pipe. The above methodology was successfully used for predicting drag reduction scale-up for surfactant-water and polymer-oil systems and heat transfer reduction for surfactant-water and polymer-water systems. The Model predictions were compared with other models available in published literature.;The use of drag reducing surfactant additives in heating or cooling systems causes an unwanted reduction in convective heat transfer in heat exchangers. This adverse effect can be overcome by intentionally breaking the heat transfer reducing surfactant micelles at the inlet to the heat exchanger. The micelles would recover further downstream and regain their drag reducing properties. A preliminary experimental investigation was conducted to study micellar break-up and recovery in a pipe. It was observed that the recovery length of micelles for a given bulk velocity was constant and was independent of pressure drop across the shearing device. A methodology, which assumes the test pipe to be a plug flow reactor, has been presented for calculating the recovery time of micelles.