Design Of Thermal Mass Flowmeter

In the modern industry production process, it is needed to measure various physics parameters so that production operation and control can be carried out effectively. Among those parameters, the fluid flowrate is an important one which is frequently measured and controlled. Because of the complexity of flowrate measurement, flowrate measurement instrument has been developed into multiple types. Thermal flowmeter is based on the principle of heat transference. At present, thermal profile flowmeter and intrusion-type thermal flowmeter are applied more generally. Thermal flowmeter is able to measure very low flowrate and the pressure loss is quite small. As it has no movable components, thermal flowmeter works very reliably.This thesis presents a method to measure gas mass flowrate in the pipeline by temperature difference of two thermistors, one of which is heated with constant power. In order to apply thermal flowmeter to liquid flowrate measurement, a cooling-type liquid mass flowmeter is also studied in this thesis. Main research work is generalized as follows:1 The development of flowmeter is summarized. The species of thermal flowmeter and their measurement principles are discussed in detail.2 The basic concepts in hydrodynamics, convection heat transfer and thermoelectricity refrigeration involved in the measurement principle of thermal flowmeter are introduced. Mathematic descriptions and analyzing means of these physics phenomena are also illustrated. The flow equations of heat-dissipation thermal flowmeter and cooling-type liquid flowmeter are deduced. And the feasibility of the two measurement methods is demonstrated.3 Computer simulation is performed in the situation of air passing by a heating columned thermistor through a round pipeline. The fluid velocity and temperature distributions under different mass flowrate are shown. They reflect the impact the heating thermistor has on the fluid. The velocity distribution on the surface of the heating thermistor is studied and the impact of natural convection is revealed. The lower limit of measurement based on the simulation result is given. The flow equation of heat-dissipation thermal flowmeter with constant power is validated through simulation data.4 The measurement circuits for each flowmeter are designed andexperiment platform based on microcontroller is set up. Statistical method combined with average filtering method is put forward to perform data filtering. By dynamically adjusting the microcontroller's power management mode and system clock, the energy consumption can be reduced conspicuously.5 Experiment results are analyzed to determine the measurement uncertainty. The uncertainty of the microcontroller's voltage input is calculated based on the A standard uncertainty assessment method. How to calculate the uncertainty of the mass flowrate is shown as well.