A comparative study on thermohydraulic performance of multi-channel spoilers and staggered spoilers

Abstract

Energy crisis has set back the global progress on economic development, because the non-renewable energy sources are unsustainable, and renewable energy sources are not yet reliable. Therefore, improving the efficiency of energy usage becomes increasingly significant. Heat is the largest energy end-use in the world, taking good advantage of it is one of the most important subjects. Tubular heat exchangers are widely used in both civil and industrial processes, and the consumption of energy can be greatly reduced by improving the efficiency of heat transfer in pipes. Adding inserts into the pipes is a main way to enhance heat transfer. This paper mainly studies two new types of spiral spoilers, namely multi-channel spoilers and staggered spoilers, using numerical methods. The former divides the pipe into several channels, and the latter has deflecting angles at the end of every spiral period. Experiments were also conducted to verify the numerical results. Aiming at analyzing thermohydraulic characteristics of fluid flow in pipes inserted with two types of spoilers separately, the effects of channel number (N), deflecting angle (φ), pitch ratio (p’/D) were studied under different Reynolds numbers (Re). Evaluation parameters including Nusselt number (Nu), friction factor (f), heat deficit loss (eT), mechanical deficit loss (ep), exergy loss (E*) and performance evaluation criteria (PEC) were all considered as indices. Results show that flow channels and deflecting angles generate better mixed fluid, which is conducive to heat transfer enhancement at the expense of more irreversible loss. Multi-channel spoilers with a channel number N = 2 and pitch ratio p’/D = 1.5 maintain the best balance between energy loss and heat transfer. The heat transfer enhancement reaches the best performance in staggered spoilers with a deflecting angle φ = 15°, 45.07% irreversible loss can be reduced compared with φ = 120°. Staggered spoilers can enhance heat transfer better, compared with multi-channel spoilers, especially in laminar flow.