ELSEVIER Sensorsand Actuators A 55 (1996) 49--55 Investigation of forced convection in microfluid systems N.T. Nguyen, D. Bochnia, R. Kiehnscherf, W. D6tzel Department of Electrical Engineering and Information Technology. Technical Universityof Chenmitz-Zwickau. 1)-09107 Chenmitz, Germany Abstract Experimental investigation, numerical simulation and analytical modelling of forced convection are presented. An electrecaloric mass-flow sensor is the object of investigations. The convective heat lost and the influence of the flow over the temperature field have been measured and calculated. The influence of flow regimes over forced convection in a micmfluid system is shown. Applications and further work are discussed at the end of the paper. Keywords: Heattransfer: Forced convection: Electrocaloric principle: Silicon sensc~s; Siliconactumors 1. Introduction Microfluid technology arises as a result of the development of microsystem technology. In the last few years, a large number of microfluid devices and systems have been pub- lished. A review of different microfluid actuators and sensors was given in Ref. [ 1 ]. The thermal principle was often used for actuating and measuring fluid flow. Thermopneumatic and bimetallic actu- ators are two important examples of thermal actuating [ 1- 3]. Most of the reported mass-flow sensors were based on the thermal principle and on forced convection, respectively. Generally, forced convection is very important in micro- fluid systems, wherever the temperature as well as the heating power are measured and controlled. Therefore, there is a good reason to investigate this phenomenon in microfluid systems. Investigation results of an electrocaloric mass-flow sensor are presented in this paper. The content deals with the forced convection in a heated microchannel. The influences of a flow on the heating power and the temperature distribution will be shown and discussed by the help of measurement, analytical modelling and numerical simulation. 2. Object and methods ofexperimental investigations The investigated object is an electrocaloric mass-flow sen- sor based on the thermoresistive principle. The flow causes a cooling of the heated resistances at a constant heating power or an increase of the heating power at a constant heater tem- perature. The fluid flow displaces the temperature distribution on the sensor. This temperature displacement can be detected 0924.4247/96/$15.00 © 1996Elsevier ScienceS.A. All fightsreserved Pll S0924-4247 (96) 0 i 249-6 by integrated temperature sensors positioned in the lower and upper parts of the heater. The operation modes and thls sensor's technology were reported in Ref. [4]. In t,~i~ paper, we concentrate on the microchannel and the heater structure. The microchannel is made by using anisotropic etching in silicon. The channel is covered by anodic bonding on a glass plate or glue directly on a carrying substrate. The channel cross section has a typ- ical trapezoidal shape. The heater structures were located on a thin diaphragm that has a thickness of 15 to 30 p.m. Three silicon resistors are in use. They are wired in parallel because of the low resistance and large contact area of the heater structure. The electrical supply is realized by using wire bond- ing contacts. The geometry of the three-heater structure is given in Fig. 1. 3. Defmifion of dimensionless parameters Classic investigation methods like din~nsio~d analysis, scaling laws and balance equations are possible in solving problems in microfluid systems with viscous fow. The effects of forced convectinn result from transfer proc- esses in fluid flow. Table i shows the transfer variables and their balance equations that help to describe and solve prob- lems in a fluid flow [7]. Because of the trapezium-like shape, the channel cross section can be characterized by using the hydraulic diameter: Dh=4AIU (1) The flow in the microchannel ranges between 0 and 250 ml min- t. That equals average velocities of approximately 0