Resonance-driven heat transfer enhancement in a natural convection boundary layer perturbed by a moderate impinging jet

We report a resonance-driven convective heat transfer enhancement downstream of a natural convection boundary layer that is perturbed by a moderate impinging jet. Flow resonance is experimentally and numerically confirmed between the boundary layer and a periodic flow driven by the unbalance of jet momentum and buoyancy. When the oscillation frequency in the impinging region is a submultiple of the characteristic frequency of natural convection, downstream heat transfer enhancement $E$ from the baseline of natural convection exceeds 40%. In contrast, further increasing the jet momentum fourfold yields $E<20\%$. The results support the development of efficient heat transfer enhancement methods.

Figure 1

Heat transfer enhancement of natural convection $\left({\mathrm{Ra}}_y<1.75\times {10}^9\right)$ by an impinging planar jet ($\mathrm{Re}<400$). (a) Schematic of the physical problem, consisting of natural convection from an isothermal vertical wall on which the jet impinges near the leading edge. The inset focuses on the impinging region where the unbalanced forces of buoyancy and momentum can cause flow instability. (b) ${\mathrm{Nu}}_y$ as a function of ${\mathrm{Ra}}_y$ for natural convection and three jet $\mathrm{Re}$. (c) Dependence of heat transfer enhancement $E$ on $\mathrm{Re}$ in two regions: zone 1 (Z1) for ${\mathrm{Ra}}_y<2.66\times {10}^8$, and zone 2 (Z2) for ${\mathrm{Ra}}_y=2.66\times {10}^8$ to $1.75\times {10}^9$; zone 3 (Z3) comprises both zones. The large $E$ at moderate $\mathrm{Re}$ and its associated cause (resonance) is the main focus of this study. The color bar shows the rms velocity probed at $y=700\mathrm{mm}$.

Figure 2

Visualized instantaneous temperature fields after 300 s from the start of impinging jet. Numerical results over the entire heated wall for (a) $\mathrm{Re}=75$, (b) $\mathrm{Re}=100$, (c) $\mathrm{Re}=150$, (d) $\mathrm{Re}=350$. Instability waves of the thermal boundary layer over the heated surface were observed for $\mathrm{Re}=100$ (also moderately at $\mathrm{Re}=250$ in Video S1). The comparison between experiments (red interferograms) and simulations around the impinging region are shown in (e) when $\mathrm{Re}=100$, and (f) $\mathrm{Re}=350$ (Video S2 shows more comparisons).

Figure 3

Thermal boundary layer analysis at three different locations: $x=2\mathrm{mm}$ and $y=130\mathrm{mm}$ (bottom/green), 400 mm (middle/blue), and 700 mm (top/red). Numerical simulation results showing the temperature time series (a) and power spectra (b) for $\mathrm{Re}=100$. Experimental results (gray background) for thermal boundary layer fluctuations (c) and power spectra (d) for $\mathrm{Re}=100$ at the same locations. Simulation results showing the power spectra for $\mathrm{Re}=150$ (e) and $\mathrm{Re}=300$ (f); Fig. S8 shows the temperature time series. The dotted (a),(c) and solid (b),(d)–(f) lines represent probed values and power spectra, respectively.