1. Introduction
Heat transfer enhancement technologies have drawn much more attentions in recent years for the intense demand in the industrial fields. Generally, the machining of rough or extended surfaces and vortex generators, as well as the promotion of working fluid properties is considered as the passive methods. While the active methods such as forced channel flow, impinging jets, mechanical vibration and electromagnetic field consume extra energy.
Among these heat transfer enhancement methods, impinging jets have the highest known single phase local heat transfer rate but low drop pressure, which is attractive for electronic thermal management [
1]. Basic parameters such as Reynolds number and jet-to-surface spacing significantly influence flow fields and heat transfer performances [
2,
3,
4]. What’s more, the further heat transfer enhancement of impinging jets can be obtained by combining other active or passive methods, which has been widely investigated by many scholars [
5,
6,
7,
8]. For example, single jet/jet arrays impinging onto dimpled target surface are numerically investigated [
9]. The associated results show that the average Nusselt number increases with the variations of dimple relative depth. Selimefendigil et al. [
10] studied the effects of pulsating frequency, Reynolds number and nanoparticle volume fraction on the fluid flow and heat transfer characteristics. Shojaeizadeh et al. [
11] analyzed the bed roughness effects on the variations of turbulent confined wall jets. Geng et al. [
12] conducted a series of experiments about unsteady impinging jets with different waveforms. Yarmand et al. [
13] analyzed the entropy generation in the square cross section tube, which had a constant heat flux under turbulent flow and nanofluids condition. Li et al. [
14] numerically studied single jet impinges on dimpled surface using Al
2O
3-Water nanofluids. They found that the average Nusselt number increases with the variations of nanoparticle volume concentration (
φ). From the previous literatures, it can be concluded that combinations of several heat transfer enhancement technologies have become one heated topic. Impinging jets with unconventional working fluid seems to be potential for strengthening heat transfer.
Non-Newtonian fluid has drawn some attentions in recent years for its obvious advantages of drag reduction and heat transfer enhancement in channel flow [
15,
16,
17,
18]. So, via Non-Newtonian fluid, impinging jet has been studied. For example, Maleki et al. [
19,
20] investigated the characteristics of the flow and heat transfer of non-Newtonian nanofluids in the porous surface and they also analyzed the features of viscous dissipation and heat absorption and generation under heat transfer and fluid flow of pseudo-plastic nanofluid condition. Poh et al. [
21] investigated a single axis-symmetric semi-confined laminar impinging jet with CMC (carboxymethyl cellulose) solutions. Their results showed that for a fixed generalized Reynolds number, when power-law index decreases, the increase of inlet velocity leads to the growing of Nusselt number. For a fixed inlet velocity, Nusselt numbers remain unchanged regardless of the changes of Reynolds number and CMC concentration. Hassan et al. [
22] deeply discussed the generation of entropy in the flowing Nanofluid fluid which was employed in the micro and minichannels. Srisamran et al. [
23] numerically simulated the flow and mix behavior of two 2D steady confined laminar impinging jets using shear-thinning fluid CMC solutions. They found that jet interaction in the impingement zone increased and the size of recirculating bubbles enlarged with the increase of
Re. Hooshmand et al. [
24] numerically investigated the effects of magnetic fields on the variations of heat transfer and generation of entropy over an axisymmetric stretching plate structure. Zhao et al. [
25] theoretically examined the spread of shear-thinning and shear-thickening liquid jet over a horizontal plate, which indicated that highly viscous and strongly shear-thinning fluids difficultly spread, even compared with highly viscous and simultaneously highly shear-thickening fluids, which results from the interaction between the shear-thinning character and the overall viscosity of the fluid. Abbas et al. [
26] discussed the generation of entropy of the peristaltic nanofluids which flowed in the channels, having the compliant walls. Yilbas et al. [
27] studied the entropy characteristics of the Non-Newtonian fluid which flowed in one annular pipe under constant viscosity condition. Shojaeizadeh et al. [
28] employed finite volume method to investigate the laminar mixed convection heat transfer of power-law non-Newtonian fluids in square enclosures. Gharraei et al. [
29] numerically studied the non-Newtonian multiple impinging jets at the Reynolds number 100–200, power-law index 0.4–1.6 and dimensionless jet-to-plate spacing 0.25–1.0. They found that under the same Reynolds number and consistency coefficient condition, with the increase of power-law index, wall Nusselt number increased because of the increasing of inlet velocity. Yousefi-Lafouraki et al. [
30] investigated entropy generation with non-Newtonian nanofluid of a confined laminar slot impinging jet. The working fluid is a CMC solution with TiO
2 nanoparticles, exhibiting pseudoplastic behavior. Overall, non-Newtonian fluid whose viscosity is sensitive to shear strain rate may be effective on impinging jets with manifest shear flow.
In the review of published literatures, the combination studies of impinging jets and non-Newtonian fluid are necessary for heat transfer enhancement and the researches especially for the studies of the heat transfer characters on a confined impinging slot jet by employing power-law liquid are relatively few. More work is still needed to obtain better heat transfer performance. In this study, a confined impinging slot jet was numerically investigated considering the effects of power-law index n, consistency index K and inlet velocity u. Moreover, industrial-fluid solutions such as CMC and xanthane are also examined. By this study, several variation tendencies of Nusselt number with diverse physical parameters can be obtained so that the desirable working fluid can be found.