Experimental investigation of flow pulsation waveforms in rectangular mesochannels for high heat flux electronics cooling

Abstract

The ever rising heat fluxes encountered in electronic devices present a challenging thermal engineering problem. Not only is heat transfer enhancement a goal but more recently control of the enhancement is desired, especially in large scale data facilities where the heated outlet coolant fluid may be of use. Flow pulsation is one of the most common active heat transfer enhancement methods studied across all heat sink sizes. While the effects of pulsation have been shown to increase heat transfer under certain conditions there is still disagreement upon its merit for practical applications, with the majority of previous research being limited to only sinusoidally oscillating flows. In this work the effect of symmetric and asymmetric excitation waveforms on heat transfer with single phase pulsatile fluid flow in mesochannels is investigated through micro particle image velocimetry (μ-PIV) and a thermal test rig. The channel measures 25 mm, 0.58 mm and 1.15 mm in length, width and height respectively, giving a hydraulic diameter (Dh) = 771 μm. A μ-PIV stage with a Nikon 10x/0.3 objective coupled to a double pulsed Nd:YLF high-speed laser and CMOS camera (1024 × 1024 pixels, 12 bit) were used for recording PIV images. The ability to operate both the camera and light source at high speeds (500 Hz) allowed for the discretization of the flow velocity over one cycle to calculate the fluctuating velocity components. For a constant Reynolds number of 150, a range of excitation frequencies were studied (1 Hz, 5 Hz, 16.55 Hz, 25 Hz) to give Womersley numbers of; 1, 2.24, 4.1, 5 respectively. To study the effect of altering excitation waveforms on heat transfer in mesochannels a high power density heater cartridge was placed below the test channel, with five thermocouples placed evenly along the entire channel length. The use of asymmetric excitation waveforms were found to generate significantly larger fluctuating velocity components in streamwise direction over symmetric waveforms. For cases with large asymmetric fluctuations the temperature of the five channel probes was found to be notably lower for a same heating power, indicating an enhancement in heat transfer. The best enhancement in heat transfer was found to be for an asymmetric waveform F(1), with a 28% increase over standard steady flow. This simple method of altering excitation waveform adds further control to the task of heat transfer in electronics cooling, where often frequency was the only previous control variable.