Heat flux sensor | EPFL Graph Search

A heat flux sensor is a transducer that generates an electrical signal proportional to the total heat rate applied to the surface of the sensor. The measured heat rate is divided by the surface area of the sensor to determine the heat flux. The heat flux can have different origins; in principle convective, radiative as well as conductive heat can be measured. Heat flux sensors are known under different names, such as heat flux transducers, heat flux gauges, or heat flux plates. Some instruments are actually single-purpose heat flux sensors, like pyranometers for solar radiation measurement. Other heat flux sensors include Gardon gauges (also known as a circular-foil gauge), thin-film thermopiles, and Schmidt-Boelter gauges. Usage Heat flux sensors are used for a variety of applications. Common applications are studies of building envelope thermal resistance, studies of the effect of fire and flames or laser power measurements. More exotic applications include estimation

Flow Boiling Pressure Drop and Heat Transfer of Refrigerants in Multi-microchannel Evaporators under Steady and Transient States

Houxue Huang

Multi-microchannel evaporators used for the cooling of high heat flux electronics have been of interest to both industry and academia for more than a decade. Such interest has sparked a large number of research studies on the flow boiling pressure drop and heat transfer in multi-microchannel evaporators. However, there are still several aspects that need to be addressed in order to better understand the complicated flow boiling process taking place in such micro-evaporators. Firstly, the mechanism governing flow boiling heat transfer in microchannels is arguable; secondly, the availability of fine-resolution local heat transfer data is very limited; thirdly, over-simplified heat conduction models are used in the literature to reduce such local heat transfer data; finally, rare attention has been taken on the thermal behavior of such micro-evaporators under transient status.\ Inspired by the forgoing aspects, an extensive experimental program has been conducted to study the flow boiling pressure drop and heat transfer of refrigerants in multi-microchannel evaporators under steady and transient status. For the steady-state tests, three fluids ( R245fa, R236fa and R1233zd(E)) were tested in two multi-microchannel evaporators. The silicon microchannel evaporators were 10 mm long and 10 mm wide, having 67 parallel channels, each 100

\times

100

\mu

^2

, separated by a fin with a thickness of 50

\mu m

. Two types of micro-orifices (25 and 50

\mu

^2

in width) were placed at the entrance of each channel to stabilize the two-phase flow and to obtain good flow distribution. The test section backside temperatures were measured by a self-calibrated infrared (IR) camera. The operating conditions for stable flow boiling tests were: mass fluxes from 1250 to 2750 kg~m

^{-2}

^{-1}

, heat fluxes from 20 to 64 W cm

^{-2}

, inlet subcoolings of 5.5, 10 and 15 K, and nominal outlet saturation temperatures of 31.5, 35 and 40

^{\circ}\mathrm{C}

. The resulting maximum exit vapor quality at the outlet manifold was 0.51. \ The steady-state experimental data were reduced by solving a 3D inverse heat conduction problem to obtain the local heat transfer coefficients on a pixel-by-pixel basis. The required fluid temperature in the subcooled region was calculated from the local energy balance, while that in the saturated flow boiling region came from the general pressure drop model proposed in this manuscript based on the present data base. According to the present data base of fine-resolution local heat transfer coefficients, a new flow pattern based prediction model was developed here starting from the subcooled region all the way through the annular flow regime. This new flow pattern based model predicted the total local heat transfer database (1,941,538 local points) well with a MAE of 14.2% and with 90.1% of the data predicted within

\pm

30%. It successfully tracks the experimental trends without any jumps in predictions when changing flow patterns.\ For the transient tests, an extensive experimental study was conducted to investigate the base temperature response of multi-microchannel evaporators under transient heat loads, including cold startups and periodic step variations in heat flux using two different test sections and two coolants (R236fa and R245fa) for a wide variety of test conditions. In addition, a transient flow boiling test under a heat flux disturbance was performed, and a new method of solving the transient 3D inverse heat conduction problem was proposed to obtain the local transient flow boiling heat transfer coefficients.

EPFL2016

Dynamic Modeling and Experimental Evaluation of a Controlled Two-Phase On-Chip Cooling System Designed for High Efficiency Datacenters

Nicolas Lamaison

An important challenge for the next generation of high performance datacenters is to guarantee sufficient, reliable, green and cheap cooling of the server blades. Since this seems unrealistic with air cooling technologies due to the expected ever higher chip heat fluxes (in excess of 100 W/cm2), a novel more effective cooling system is required. Switching to two-phase on-chip cooling systems would decrease the energy consumption, enhance the thermal performance and allow the reuse of the extracted heat. However, the design of such systems seems nontrivial because of the lack of simulation tools able to reproduce their behavior. In this context, the present thesis describes the one-dimensional dynamic modeling of an on-chip two-phase liquid pump cooling system for multiple micro-processors. The simulation code includes state-of-the art two-phase flow methods, two-dimensional conduction in the heat sink, heat flux dependent mass flow rate distribution between parallel micro-evaporators, and micro-condenser and in-line liquid accumulator models. Three different scales of simulations, namely the heat sink, multiple parallel heat sinks and entire system scales, are presented. An experimental test bench with a cooling capacity of about 300W is used to validate the code. Mass flow rate distribution, energy oriented performance maps, chip temperature profile, heat flux disturbances under controlled conditions and charge influence are studied experimentally. The experimental data are satisfactorily reproduced by the simulations with, for example, 266 steady-state mean chip temperatures, obtained for balanced and unbalanced heat fluxes conditions, predicted within ± 2K. The most significant discrepancy between simulations and experiments is found to be the pressure transient prediction. The validated code is then used to simulate a compact two-phase on-chip cooling system designed for a real datacenter blade server. At the heat sink level, start-up and hot-spots are studied using 2D temperature maps and time-strip analysis. Furthermore, the flow distribution between the parallel micro-evaporators of the studied layout is shown to be unique and thermal-hydraulic maps are generated. The system start-up and operation under random and periodic heat load disturbances with different control strategies are then analyzed. Finally, the thermo-economic integration of a datacenter with a district heating network is investigated.

EPFL2014

Experimental characterisation of the thermohydraulic performance of a micro-pin fin evaporator: Heat transfer, flow visualization and modelling

Chiara Falsetti

The global tendency towards miniaturization driven by the microelectronics industry is pushing system density and packaging towards unprecedented values of thermal design power, with a dramatic reduction of the surface area of the devices. Vertical integration, which consists of single dies stacked and connected by means of Through-Silicon-Vias (TSVs), is a promising architecture, but it is hampered by the requirement of finding innovative and more effective cooling technology for dissipating the greater amount of heat. On-chip interlayer two-phase cooling represents a very attractive long-term solution for this problem. However, the design of a two-phase-based cooling system and the ability to predict its performance depend on both the availability of high-accuracy experimental data and appropriate models. The focus of the present study is thus to provide a broad experimental database to describe the physical processes associated with two-phase flow and characterise the thermal and hydraulic performances of a new micro-pin fin evaporator, in order to assess its feasibility for the realization of a two-phase-based cooling technology. The micro-evaporator tested here has a heated area of 1cm^2, consisting of 66 rows of cylindrical in-line micro-pin fins with a diameter, height and pitch of respectively 50 micron, 100 micron and 91.7 micron. Channel entrances with and without inlet restrictions are tested in order to demonstrate the beneficial stabilizing effect of placing an extra row of micro-pin fins with a larger diameter of 100 micron at the beginning of the flow passage area. Insights on the local trends and magnitudes of the heat transfer coefficient and pressure drops were gained over a wide range of test conditions with mass flux varying from 500 kg/(m^2s) to 2500 kg/(m^2s), heat flux ranging from 20 W/cm^2 to 48 W/cm^2, and constant outlet saturation temperatures of 25 °C, 30.5 °C and 35 °C. The tested four refrigerants were R236fa, R245fa, R1234ze(E) and R134a. Based on the experimental observations, the heat transfer coefficient trend enhances significantly with the applied heat and the vapor quality, while mass flux exhibits a slightly minor influence. These results evidence the mutual influence between the thermal performance of the micro-evaporator and the flow pattern development during its operation, which has been further investigated by synchronizing the infrared temperature measurements and the high speed flow visualization. Moreover, new insights on the nucleation process and on the interface dynamics were gained by applying image processing and time-strip technique to the high-speed camera videos. Finally, a new heat transfer prediction method that focused on the physical understanding and predictive capability of flow boiling in between micro-pin fins was developed and compared with the obtained experimental database (7219 local measurements). The new model incorporates the complex flow geometry and the characteristics of the flow regimes occurring along the micro-evaporator, and it successfully predicted the experimental data.

EPFL2018