School of Mechanical Engineering Seminar
Thursday, March 6, 2019 at 14:00
Wolfson Building of Mechanical Engineering, Room 206
Finding the Limits of Submerged Jet Heat Transfer
Barak Kashi
PhD student of Herman Haustein
Impinging jets offer much higher transport rates than other canonical flows and therefore have many industrial applications. Specifically, for cooling of electronic components they are often employed in arrays of micro-jets for improved uniformity. Examination of the literature shows that prior to dealing with arrays, key understandings of single jet heat transfer are still lacking. Within this study, these gaps are filled by new relations which are developed and validated against numerical simulations over a wide range of working conditions.
While traditional macro-scale heat transfer (HT) correlations predict continuous increase in HT during down-scaling, it is here found that this trend reverses at the micro-scale due to viscous dissipation even under low-viscosity laminar flow conditions. Thus, a maximum exists for effective cooling, beyond which any further down-scaling decreases the cooling rate. Furthermore, It is shown that when dissipation becomes significant, the effective HT is no longer independent of the imposed heat flux and the heating-cooling symmetry is broken.
Next, dependencies of HT on geometrical parameters such as nozzle length and nozzle-to-wall distance are resolved, showing that the velocity profile arriving at the wall uniquely determines the stagnation-point HT. First examining short-nozzle flow reveals three distinct flow regimes, associated with flow separation at the inlet and its reattachment. The corresponding non-monotonous dependence of HT on nozzle length is then correlated for short nozzle-to-wall distances. Larger nozzle-to-wall distances are here dealt with by deriving an approximate solution to the axial momentum equation, which gives a good description of the jet core for a wide range of flight distances and for all issuing profile. This approximate solution, not only revealed the true form of the jet’s potential core, but also led to the identification of a new concept – the “boundary core” delimiting the jet’s interaction with its surroundings.
Finally, a mechanistic model is developed which provides the HT distribution under the jet and its relation to the arriving jet profile. Therein, apparent contradiction in observed stagnation-point HT trends is resolved, while the non-monotonous radial distribution under quasi-uniform jets is explained.
The contribution of the present work is two-fold: i) identifying and quantifying the inherent lower bound of beneficial down-scaling of jet impingement systems – thus curbing the on-going trend of jet-array miniaturization; ii) the relations developed here significantly extend jet impingement theory and enable finding the optimal HT regardless of scale.
