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ABSTRACT
An experimental setup was built to study the influence of conduction heat loss on the convective heat transfer performance enhanced by an impinging jet in cross-flows. Results revealed that the conduction heat loss ratio (Ec/E) is between 12.0% and 40.1%, and it decreases nonlinearly with the ratio of jet-to-cross-flow velocity. The relative Nusselt number increases with the ratio of jet-to-cross-flow velocity. The maximum peak value and the average are 8.1 and 6.4, respectively. The distribution of the relative Nusselt number seems to be flattened by assuming a constant conduction heat loss ratio.
Nomenclature
| A | = | Constant |
| A0 | = | Area of the target surface (m2) |
| Ap | = | Pixel area (m2) |
| As | = | Total outside area of the HTCC heater (m2) |
| B | = | Constant |
| Bi | = | Biot number (= haveV/Aks) |
| CVP | = | Counter-rotating vortex pair |
| d | = | Diameter of the impinging jet (m) |
| D | = | Hydraulic diameter of the cross-flow channel (m) |
| E | = | Heat input power (W) |
| Ec | = | Conduction heat loss (W) |
| Ecp | = | Local conduction heat loss (W) |
| hp | = | Local convective heat transfer coefficient (W/m2-K) |
| hp,0 | = | Local convective heat transfer coefficient without impinging jet (W/m2-K) |
| hy | = | Local convective heat transfer coefficient in y direction (W/m2-K) |
| HFS | = | Heat flux sensor |
| HTCC | = | High temperature co-fired ceramic |
| JICF | = | Jet in cross-flow |
| k | = | Thermal conductivity of air (W/m-K) |
| ks | = | Thermal conductivity of the HTCC (W/m-K) |
| L | = | Distance between the jet nozzle and the HTCC heater (m) |
| N | = | Pixel number |
| Nuy | = | Local Nusselt number (=hy(y+y0)/k) |
| Nur,ave | = | Averaged relative Nusselt number |
| Nur,p | = | Local relative Nusselt number |
| Nur,pk | = | Peak relative Nusselt number |
| Pr | = | Prandtl number |
| q | = | Input heat flux (W/m2) |
| qc | = | Averaged conduction heat flux (W/m2) |
| r | = | Jet-to-cross-flow velocity (=uj/uc) |
| ReD | = | Reynolds number (=VcρD/μ) |
| Rej | = | Reynolds number (=Vjρd/μ) |
| Rey | = | Reynolds number (=Vcρ(y+y0)/μ) |
| T | = | Temperature (K) |
| Tave | = | Averaged surface temperature (K) |
| T0 | = | Inlet air temperature (K) |
| Tmon. | = | HFS temperature (K) |
| Tp | = | Pixel temperature (K) |
| Ts | = | Surrounding temperature (K) |
| uy | = | Velocity component in y direction (m/s) |
| V | = | Volume of the HTCC heater (m3) |
| Vc | = | Cross-flow velocity (m/s) |
| Vj | = | Impinging jet velocity (m/s) |
| x | = | Coordinate (m) |
| y | = | Coordinate (m) |
| y0 | = | Starting point of the HTCC heater (m) |
| z | = | Coordinate (m) |
Greek symbols
| ε | = | Radiation emissivity |
| μ | = | Dynamic viscosity (Pa·s) |
| σ | = | Stefan–Boltzmann constant (W/m2K4) |
| ρ | = | Air density (kg/m3) |
| δ | = | Uncertainty |
| ξ | = | Multiplication factor |