3.1 Fig. 1 is a sectional view of an example circular foil heat-flux transducer. It consists of a circular Constantan foil attached by a metallic bonding process to a heat sink of oxygen-free high conductivity copper (OFHC), with copper leads attached at the center of the circular foil and at any point on the heat-sink body. The transducer impedance is usually less than 1 V. To minimize current flow, the data acquisition system (DAS) should be a potentiometric system or have an input impedance of at least 100 000 Ω.
3.2 As noted in 2.3, an approximately linear output (versus heat flux) is produced when the body and center wire of the transducer are constructed of copper and the circular foil is constantan. Other metal combinations may be employed for use at higher temperatures, but most (4) are nonlinear.
3.3 Because the thermocouple junction at the edge of the foil is the reference for the center thermocouple, no cold junction compensation is required with this instrument. The wire leads used to convey the signal from the transducer to the readout device are normally made of stranded, tinned copper, insulated with TFE-fluorocarbon and shielded with a braid over-wrap that is also TFE-fluorocarbon-covered.
3.4 Transducers with a heat-sink thermocouple can be used to indicate the foil center temperature. Once the edge temperature is known, the temperature difference from the foil edge to its center may be directly read from the copper-constantan (Type T) thermocouple table. This temperature difference then is added to the body temperature, indicating the foil center temperature.
3.5 Water-Cooled Transducer:
3.5.1 A water-cooled transducer should be used in any application where the copper heat-sink would rise above 235°C (450°F) without cooling. Examples of cooled transducers are shown in Fig. 2. The coolant flow must be sufficient to prevent local boiling of the coolant inside the transducer body, with its characteristic pulsations (“chugging”) of the exit flow indicating that boiling is occurring. Water-cooled transducers can use brass water tubes and sides for better machinability and mechanical strength.
3.5.2 The water pressure required for a given transducer design and heat-flux level depends on the flow resistance and the shape of the internal passages. Rarely will a transducer require more than a few litres of water per minute. Most require only a fraction of litres per minute.
3.5.3 Heat fluxes in excess of 3400 W/cm28201;(3000 Btu/ft2/s) may require transducers with thin internal shells for efficient transfer of heat from the foil/heat sink into a high-velocity water channel. Velocities of 15 to 30 m/s (49 to 98 ft/s) are produced by water at 3.4 to 6.9 MPa (500 to 1000 psi). For such thin shells, zirconium-copper may be used for its combination of strength and high therm......