Date of Award

Summer 2019

Document Type

Open Access Thesis

Department

Mechanical Engineering

First Advisor

Jamil A. Khan

Abstract

Continual growth of hydraulic and thermal boundary layers along stream wise direction in conventional straight fin mini-channel heat sink causes gradual deterioration of their thermal performance. To enhance thermal-hydraulic performance by breaking and redevelopment of the boundary layers, this research aims to introduce a novel inter-connected mini-channel sinks. Two inter-connectors were positioned transversely between two adjacent mini-channels, which segmented the flow domain into three zones. Secondary flow was generated through the inter-connectors utilizing the pressure difference of the adjacent channels resulting in hydraulic and thermal boundary layers disruption and hence enhanced thermal-hydraulic performance of the mini-channel heat sink was achieved.

Firstly, this present work attempts to numerically analyze and compare the effects of inter-connectors width on the heat transfer and fluid flow behaviors of parallel and counter flow mini-channel heat sinks. Five different inter-connector width (case 1-5) were considered for a fixed inter-connector location (zones length). A corresponding conventional parallel flow mini-channel heat sink was chosen as the base case in contrast to the newly proposed inter-connected mini-channel heat sinks. The length, height, and width of the considered mini-channel heat sink were 26 mm, 0.5 mm, and 1.5 mm respectively, which provides a hydraulic diameter of 750 ΞΌm. Water was employed as the coolant, and the flow was in the single-phase regime under laminar flow condition at Reynolds numbers (𝑅𝑒) ranging from 150 to 1044. The non-dimensional pressure, velocity temperature, friction factor, overall Nusselt number (𝑁𝑒), and thermal resistance were calculated to evaluate the overall performance of the inter-connected mini-channel heat sink. Finally, the performance of the inter-connected mini channel was compared with the conventional parallel flow mini-channel by calculating the performance evaluation criteria (PEC). The results show that the inter-connector has negligible effect on the overall performance of the parallel flow mini-channel heat sinks because of the almost no transverse flow through the inter-connectors whereas, inter-connector has significant effect on the overall performance of the counter flow mini-channel heat sinks. For the counter flow mini-channel heat sink and for the highest considered inter-connectors width (case 5), Nu was enhanced by a maximum of 36% at 𝑅𝑒=1044 as compared to the conventional parallel flow mini-channel while a maximum of 31.13% reduction in friction factor was recorded at 𝑅𝑒=150. The PEC of the inter-connected counter flow mini-channel heat sink (case 5) went up to 1.33, and its value shows an increasing trend with as 𝑅𝑒 increases.

Secondly, to examine the combined effect of the inter-connectors width and location, i.e., zones length on the thermal-hydraulic characteristics of the counter flow mini-channel heat sink, the present numerical studies were carried out for nine different cases (case 1-9) by varying inter-connectors width and location. The results show that the amount of secondary flow reduces gradually as 𝑅𝑒 increases for any particular inter-connectors location and width. At the lowest considered 𝑅𝑒 (𝑅𝑒=150), a maximum value of PEC was achieved to ~1.22 for the highest length of zone 1 and 3 and the lowest inter-connectors width (case 7), while at the highest 𝑅𝑒 (𝑅𝑒=1044), the maximum PEC value (~1.42) was recorded for the intermediate length of zone 1 and 3 and the highest inter-connectors width (case 6).

Thirdly, to validate the numerical predictions, experimental investigations of heat transfer and fluid flow characteristics of conventional parallel and counter flow mini-channel heat sinks and also inter-connected parallel and counter flow mini-channel heat sinks were performed under laminar flow regime. For experimental analysis, numerically obtained optimum inter-connectors width, and location was chosen as fabrication parameters. The experimental heat transfer results for all the conventional and inter-connected mini-channel heat sinks show excellent agreement with the corresponding numerical results. On the contrary, experimentally obtained pressure drop were substantially less compared to the numerically predicted pressure drop, especially at low 𝑅𝑒. Plausible reasons for the reduced pressure drop are discussed. Experimental results show that inter-connected parallel flow mini-channel heat sinks provide poor overall performance whereas inter-connected counter flow mini-channel heat sinks provide superior overall performance compared to the conventional parallel flow mini-channel heat sink.

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