Abstract
It is well known that rough surfaces affect turbulent flows significantly. How such surfaces affect turbulent heat transfer is less well understood. To gain more insight, we have performed a series of direct numerical simulations of turbulent heat transfer in a channel flow with grit-blasted surfaces. An immersed boundary method is used to account for the rough surface. A source term in the thermal energy balance is used to maximise the analogy between the transport of heat and the transport of streamwise momentum. The wall roughness size is varied from k+=15 to k+=120. Turbulence statistics like mean temperature profile, mean temperature fluctuations and heat fluxes are presented. The structure of the turbulent temperature field is analysed in detail. Recirculation zones, which are the result of an adverse pressure gradient, have a profound effect on heat transfer. This is important as it leads to the wall-scaled mean temperature profiles being of larger magnitude than the mean velocity profiles both inside and outside the roughness layer. This means that the temperature wall roughness function ΔΘ+(ks+,Pr) is different from the momentum wall roughness function ΔU+(ks+). Since the bulk temperature and velocity depend on ΔΘ+(ks+,Pr) and ΔU+(ks+), it was shown that the Stanton number and the skin friction factor directly depend on ΔΘ+(ks+,Pr) and ΔU+(ks+), respectively. Therefore, the failure of the Reynolds analogy in fully rough conditions can be directly related to the difference betweenΔΘ+(ks+,Pr) and ΔU+(ks+).
Highlights
Rough surfaces can be found in many engineering systems
Thakkar et al [36] have shown that direct numerical simulations of turbulent flow over a grit-blasted surface yields results that are very close to the results that were originally reported by Nikuradse [27]
We have analysed the effect of wall roughness on turbulent heat transfer
Summary
Rough surfaces can be found in many engineering systems. Typical causes of roughness include machining, fouling or corrosion. It is well known that wall roughness affects both the mean flow as well as the turbulent motion of a fluid, which in turn affects skin friction. A well-known result of the effects of roughness is summarised in the form of the Moodydiagram. The effect of roughness on turbulent flows is typically summarised by a single function, which is called the wall roughness function. Recent work from Busse et al [6] and Thakkar et al [35] shows that direct numerical simulations of turbulent flows over realistic surfaces can be used to help determine which topological characteristics should be used to predict the wall roughness function. Thakkar et al [36] have shown that direct numerical simulations of turbulent flow over a grit-blasted surface yields results that are very close to the results that were originally reported by Nikuradse [27]
Sign-in/Register to view highlights & summary Sign-in/Register to access full text options 
Free) 
Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a call
