Each element works in a constant temperature difference (CTD) mode. The readouts of the four sensing elements are used to deduce the flow parameters of the 2-D flow (i.e., flow speed and direction angle) using a neural network data fusion technique. Compared with previous technologies, the sensor has merits of simple structure, low cost, easy fabrication and low power consumption.Figure 1.Prototype of hot-film flow vector sensor.2.?Operation Principle and Design of the Sensor SystemThe sensor uses thermal elements serving as both Joule heater and temperature sensor so that it has a relative simple structure and low-cost fabrication.2.1. Sensing PrinciplesThe working principle of the sensor is based on the heat transfer of the heating element in a flow field [11], which forms a temperature distribution above the thermal element.

Under a constant bias power and zero flow speed, the thermal element achieves a steady-state temperature, which means the heat transfer system reaches equilibrium. When an external flow passes through the sensor, the temperature field will be deflected in the direction of the flow that results in the temperature differences among the elements according to their locations of upstream or downstream as shown in Figure 2. Temperature differences among the four elements can be detected and used to figure out the magnitude and direction of the flow.Figure 2.Temperature distribution above the surface of thermal elements.2.2. Sensing Design and SimulationFor sensing the 2-D flow in the directional range of 360��, both heating and sensing structures need to follow some requirements.

Firstly, the heating structure needs to have central symmetry so as to form a centrosymmetric temperature distribution above the sensor, specifically a circular symmetry is an optimal option for covering 360�� in all directions. Secondly, the temperature sensing structure needs to be divided into several isolated sections to detect the GSK-3 flow-induced temperature differences. For integrating the heating and sensing elements into one element, we consider the use of a round shape and divide it equally into several sections. The number of divided sections gives the number of heating/sensing elements, which also determines the number of conditioning circuits needed to operate the heating and temperature sensing.

For simplifying the operation and saving energy, the number of heating/sensing elements needs to be minimized. After overall considerations, we divide the round shape into four equal sections as shown in Figure 4, each of which is a quadrant consisting of a roundabout wire, as shown in Figures 1 and and33.Figure 3.Sensor design.Figure 4.Results of simulation under a flow with different flow directions.The sensitive area of the sensor needs to be as small as possible so as to capably detect the local flow at one point.