Effect of Bifurcation on Thermal Characteristics of Convergent-Divergent Shaped Microchannel

A microchannel heat sink with convergent-divergent (CD) shape and bifurcation is presented, and flow and heat transfer characteristics are analyzed for Re ranging from 120 to 900. The three-dimensional governing equations for the conjugate heat transfer with temperature-dependent solid and fluid properties are solved using the finite volume method. Comparisons are carried out for four cases, namely, rectangular shape with and without bifurcation and CD shape with and without bifurcation. The pressure drop, flow structure, and average Nusselt number are analyzed in detail, and the thermal resistance and overall performance are compared. It is shown that the CD shape with bifurcation has more uniform and lower temperature at the bottom wall and better heat transfer performance compared to other geometries. The heat transfer augmentation in the CD shaped microchannel with bifurcation can be attributed not only to the accelerated and redirected flow toward the constant cross section segment but also to periodically interrupted and redeveloped thermal boundary-layers due to bifurcation. It is also shown that increasing Re leads to thinning of thermal boundary-layers resulting in an enhanced heat transfer in terms of an increased average Nusselt number from 38% to 74%. However, there is an increased pressure drop due to channel shape and obstacle in fluid flow. Further, due to a high pressure drop penalty at high Re, CD shaped microchannel with bifurcation loses its heat transfer effectiveness.

Thermal Boundary Layer Effects on Line-of-Sight Tunable Diode Laser Absorption Spectroscopy (TDLAS) Gas Concentration Measurements

The effects of thermal boundary layers on tunable diode laser absorption spectroscopy (TDLAS) measurement results must be quantified when using the line-of-sight (LOS) TDLAS under conditions with spatial temperature gradient. In this paper, a new methodology based on spectral simulation is presented quantifying the LOS TDLAS measurement deviation under conditions with thermal boundary layers. The effects of different temperature gradients and thermal boundary layer thickness on spectral collisional widths and gas concentration measurements are quantified. A CO2 TDLAS spectrometer, which has two gas cells to generate the spatial temperature gradients, was employed to validate the simulation results. The measured deviations and LOS averaged collisional widths are in very good agreement with the simulated results for conditions with different temperature gradients. We demonstrate quantification of thermal boundary layers’ thickness with proposed method by exploitation of the LOS averaged the collisional width of the path-integrated spectrum.

Numerical Simulation of Heat Distribution with Temperature-Dependent Thermal Conductivity in a Two-Dimensional Liquid Flow

This paper is a contribution to a better understanding of heat transfer through porous channels used for mechanical sieving and filtration of liquids. The problem modeled by means of the Navier–Stokes equations and the energy equation is similar to a viscous flow between two uniformly permeable walls fixed at different temperatures. Thermal behaviors are determined through three branches denoted solutions of types I, II and III of a diagram of bifurcations presenting the values of the wall shear stress as the Reynolds number varies. We found that the distribution of temperature is similar through branches I and II where a large horizontal inflection area is observed as the Péclet number increases. This large horizontal inflection area inside the channel denotes the presence of thermal boundary layers which more precisely occur across branches I and II when the Péclet number approaches the value of 10. On the other hand, along branch III, thermal boundary layers do not exist and temperature presents a different behavior compared to those of branches I and II.