Mehran Ahmadi

Natural Convection From Interrupted Vertical Walls

Steady-state external natural convection heat transfer from interrupted rectangular vertical walls is investigated. A systematic numerical, experimental, and analytical study is conducted on the effect of adding interruptions to a vertical plate. COMSOL multiphysics is used to develop a two-dimensional numerical model for investigation of fin interruption effects on natural convection. A custom-designed testbed is built and six interrupted wall samples are machined from aluminum. An effective length is introduced for calculating the natural convection heat transfer from interrupted vertical walls. Performing an asymptotic analysis and using a blending technique, a new compact relationship is proposed for the Nusselt number. Our results show that adding interruptions to a vertical wall can enhance heat transfer rate up to 16% and reduce the weight of the fins, which in turn, lead to lower manufacturing and material costs. [DOI: 10.1115/1.4028369]

A New Analytical Approach for Dynamic Modeling of Passive Multicomponent Cooling Systems

A new one-dimensional thermal network modeling approach is proposed that can accurately predict transient/dynamic temperature distribution of passive cooling systems. The present model has applications in variety of electronic, power electronic, photonics, and telecom systems, especially where the system load fluctuates over time. The main components of a cooling system including: heat spreaders, heat pipes, and heat sinks as well as thermal boundary conditions such as natural convection and radiation heat transfer are analyzed, analytically modeled and presented in the form of resistance and capacitance (RC) network blocks. The present model is capable of predicting the transient/dynamic (and steady state) thermal behavior of cooling system with significantly less cost of modeling compared to conventional numerical simulations. Furthermore, the present method takes into account system “thermal inertia” and is capable of capturing thermal lags in various components. The model is presented in two forms: zero-dimensional and onedimensional which are different in terms of complicacy. A custom-designed test-bed is also built and a comprehensive experimental study is conducted to validate the proposed model. The experimental results show great agreement, less than 4.5% relative difference in comparison with the modeling results. [DOI: 10.1115/1.4027509]

Natural Convection from Vertical Parallel Plates: An Integral Method Solution

Steady-state external natural convection heat transfer from isothermal, vertically mounted rectangular fins is modeled analytically. An integral technique is used to solve the governing equations. Compact relationships are developedfor thevelocityandtemperature profilesforthebuoyancy-drivenchannelflowforthe1000 ≤ Ras ≤ 4500 range. Comprehensive numerical and experimental studies are performed. The proposed analytical model is successfully validated against the numerical data with the maximum relative differences of 9.8 and 3.5% for velocity and temperature profiles, respectively. Also, new semianalytical local and average Nusselt numbers are reported as functions of the Rayleigh number, temperature difference, and fin aspect ratio; and they are compared against the experimental data with good accuracy of 1.6% maximum relative difference.

Effect of Fin Interruptions on Natural Convection Heat Transfer From a Rectangular Interrupted Single-Wall

Steady-state external natural convective heat transfer from a single-wall vertically-mounted rectangular interrupted fin arrays is investigated. A systematic numerical, experimental, and analytical study is conducted on the effect of adding interruptions to a single vertical plate, on natural convective heat transfer. COMSOL Multiphysics software is used in order to develop a two-dimensional numerical model for investigation of fin interruption effects. To perform an experimental study and to verify the analytical and numerical results, a custom-designed testbed was developed. Results show that adding interruptions to a vertical single fin enhances the thermal performance of it and reduces the weight of the heatsink, which in turn, can lead to lower manufacturing costs. A compact relationship for the Nusselt number based on geometrical parameters for interrupted fins is presented using a blending technique for two asymptotes of interruption length.

Passive cooling of outside plant power systems, a green solution to reduce energy consumption

A heatpipe integrated fully passive cooling system is designed and developed for 1.2 kW rectifiers, with application in telecom outside plant power systems. Four units of the rectifiers are assembled on a telecom enclosure and tested at 46°C ambient temperature. Results show that fully passive thermal management system with naturally cooled heatsinks is capable of dissipating the generated heat in the electrical components, efficiently. The maximum temperature on the heat generating component s reached 70°C which is 10°C below its maximum allowable temperature. A mathematical model is also developed for the designed cooling system, and the equivalent R-C model of the thermal management system is presented. The results from the mathematical model show a good agreement with experimental data.

Dynamic Heat Transfer Inside Multilayered Packages with Arbitrary Heat Generations

New compact closed-form relationships are developed for calculating 1) the temperature distribution inside multilayered media, 2) the average temperature of each layer, and 3) the interfacial heat flux. As an example, the methodology is applied to a two-concentric-cylinder composite. A detailed parametric study is conducted, and the critical values for the dimensionless parameters are evaluated; beyond these values, the temperature field inside the media is not affected considerably for any combination of other variables. It is shown that there is an optimum angular frequency that maximizes the amplitude of the interfacial heat flux. An independent numerical simulation is also performed using commercially available software ANSYS; the maximum relative difference between the obtained numerical data and the analytical model is less than 2%. Nomenclature A = matrix defined in Eq. (A5) An = constant, Eqs. (A19) and (A20) Cjn = integration coefficient, Eq. (A3) Djn = integration coefficient, Eq. (A3) F = constant, Eq. (A8) Fo = Fourier number, α1t∕x 2 G = arbitrary function of η, Eq. (A11) Gn = constants defined in Eq. (A16) H = constant, Eq. (A11)

Effects of Geometrical Parameters on Natural Convective Heat Transfer From Vertically-Mounted Rectangular Interrupted Fins

Steady-state external natural convection heat transfer from vertically-mounted rectangular interrupted fins is investigated numerically and experimentally. To perform an experimental study, a custom-designed testbed was developed to verify the analytical and numerical results. FLUENT software was used in order to develop a 2-D numerical model for investigation of interruption effects. After regenerating, and validating the existing analytical results for fin spacing, a systematic numerical and experimental study was conducted on effect of fin interruption. Results show that adding interruptions to vertical rectangular fins enhances the thermal performance of fins. In a parametric study optimum interruption length for maximum fin performance was found and correlated.Copyright

Development of a passively cooled outdoor telecom power enclosure

This paper documents the development of a passively cooled outdoor electronics enclosure consisting of six 1.2kW AC/DC rectifiers (7.2kW total output power). Commercially available fan cooled rectifiers were used as a starting point and modified to be passively cooled using heat pipes and naturally-cooled heat sinks without changing the layout of the original circuit board. Several designs were considered, tested and modified to develop a fully passive thermal solution. The prototype was tested at ambient air temperatures of 26°C, 36°C and 46°C and it delivered 98.8%, 85.7% and 80.7% of its nominal output power at each of these temperatures, respectively.

Analytical investigation of thermal contact resistance (TCR) behavior under time-dependent thermal load

Thermal contact resistance under general time-dependent thermal load is investigated in this study. Assuming known number of contacts points, and the average surface area of each contact, the focus of this study is on the thermal aspect of contact resistance, i.e. spreading/constriction resistance at the contact point. A general analytical solution for thermal spreading/constriction resistances of a time-dependent circular source on a finite circular cylinder with uniform side and end cooling is presented. The solution is applicable to a general axisymmetric heat flux distribution, including both isoflux and isothermal distributions. The time response of the flux tube under different geometrical and boundary conditions is investigated and the result are shown. The results are also compared to, and successfully verified by an independent numerical simulation.

A modified steady state method for measurement of in-plane thermal conductivity

A modified version of Parallel Thermal Conductance, Two-Length Method, for in-plane thermal conductivity measurement in slabs is presented. Two modifications have been applied to the currently available methods to improve the accuracy and repeatability of the results; first, to consider and calculate the heat loss resistance in the data analysis, and second, to design the tests in a way that can accommodate multiple samples instead of one to increase the accuracy and decrease the uncertainty of the data analysis. In the end, a semi-empirical technique, using a custom-designed test setup and an analytical solution for temperature distribution inside the anisotropic rectangular plates exposed to multiple hotspots, is proposed. In the introduced technique, using the experimental results, the in-plane thermal conductivity of the sample can be calculated with good accuracy.