Heat flux meters

Heat fluxes in fire conditions have commonly been measured by steady state (fast time response) devices namely a Schmidt-Boelter heat flux meter or a Gordon heat flux meter. The former uses a thermopile over a thin film of known conductivity, with a controlled back-face temperature the latter uses a suspended foil with a fixed edge temperature. The temperature difference between the center of the foil and its edge is directly proportional to an imposed uniform heat flux. Because the Gordon meter does not have a uniform temperature over its surface, convective heat flux may not be accurately measured. 

Note that if this net flux is for a heat flux meter cooled at 7 ,XJ, the ambient temperature, the gage directly measures the external incident radiative and flame heat fluxes. 

External radiant heat flux (q") is expressed as that measured by a heat flux meter maintained at Too. Correspondingly, the surface loss is given with respect to T as... 

The gas flow to the radiant panel is set to obtain a heat flux at the 50 mm location that is 5-10kW/m2 above the critical heat flux for ignition (Section 14.3.2.3.2). The heat flux is verified with a heat flux meter inserted into a calcium silicate dummy specimen. The heat flux profile is shown in Figure 14.6 and can be used to determine the incident radiant heat flux at any location along the specimen center-line. The flux invariant may vary slightly and is usually determined for each apparatus during the initial calibration. 

A heat flux meter attached to the inner surface of a 3-cm-lhick refrigerator door indicates a heal flux of 25 W/m through the door. Also, the temperatures of the inner and the outer surfaces of the door are measured to be 7 C and I5 C, respectively. Determine the average thermal conductivity of the refrigerator door. Answer 0.0938 W/m C... 

The specimen and the heat flux meter were fixed to a revolving arm. To obtain the required temperature of the specimen, the air mass flow and the arc current were adjusted. When the tunnel was operated under the conditions shown in Table 2, the top of the specimen was heated up to 1,840 K. The surface temperature was measured using a thermocamera. The specimen was heated for 1,100 seconds in each test. 

A commercially available instrument, used to measure the heat flux directly, is known as a plug-type heat flux meter. The incident heat is conducted through the plug of area A and conductivity k to the cooling water on the other surface. Two thermocouples embedded a distance l apart measure temperatures as T and Ti. What is the total heat incident on the plug face ... 

By using an appropriate spectral filter to block out the undesirable longer wavelength (A, > 2.7 pm) radiation, a heat flux meter can then be used to conveniently give the integrated radiation intensity that would be most penetrating into molten glass. Ji and Baukal tested... 

To estimate the radiation intensity beyond 5.0 pm, a sapphire window was placed in front of the total heat flux meter. The transmission efficiency of a sapphire window is nearly constant at about 84% for X = 0.25-5.0 pm, but quickly drops beyond 5.0 pm and cuts off at... 

Brajuskovic, B., Matovic, M., and Afgan, N. "A Heat Flux-Meter for Ash Deposit Monitoring Systems -1. Ash Deposit Prevention." International Journal of Heat and Mass Transfer 34, no. 9 (1991) 2291—2301. 

Many heat flux meters are available in the market and an appropriate type of instrument can be chosen for such measurement. The decision on sensor installation is very important because it should be easily replaceable when required. An approximation of the heat flux can be... 

Heat flux meters that measure changes in heat flux for given conditions. The reduction in heat flux is a measure of the thermal resistance of the accumulated deposit. 

An improvement has been made for the calorimeter by making the sample cylindrical in shape and long enough so that the heat flux meter is placed at mid-height of... 

Since a textile substrate is used as an auxiliary wall to create the textile heat flux-meter, die performance of this heat fluxmeter is influenced by textile substrate properties. The first part of this chapter is especially concerned with the complex phenomena of the heat and mass transfers. The following section presents the principles and the production processes of the conventional and textile heat fluxmeters. 

The insertion of the electroconductor wire, either monoconductor or biconductor, into the textile auxiliary wall was realized during the weaving process. The weaving process was followed by a posttreatment to transform the textile substrate to a heat flux-meter by obtaining the Cn/Cu thermocouple junctions (thermopile) on both sides of the textile auxiliary wall. 

Three textile heat fluxmeters, ie, PES/CO2, PES3, and PES/C03 which were produced with subtractive method or additive method, were compared to a commercial heat flux-meter for heat and mass transfer properties. 

A square sample of 100 x 100 mm is exposed to the radiant flux of an electric heater. The heater has the shape of a truncated cone (hence the name of the instrument) and is capable of providing heat fluxes to the specimen up to 100 kW/m. An electric spark plug is used for piloted ignition. Heater temperature is measured as an average of the readings of three thermoconples in contact with the coil. It is set and maintained at a certain level by a three-term controller. Calibration of heat flnx as a function of heater temperatnre is performed with a total heat flux meter of the Schmidt-Boelter type. Prior to testing, the heater temperatnre is set at the appropriate value resulting in the desired heat flux. 

Guarded heat flow meters are variants on the guarded hot-plate technique, which utilize a heat flow meter to measure the heat flux. These techniques are comparative in nature because a reference material is needed to calibrate the heat flux meter. They utilize smaller samples and are thus able to equilibrate faster, improving the versatility of the guarded hot-plate method. 

Space craft are exposed to extreme conditions on reentry into the atmosphere. It is of interest to investigate the thermal effects quantitatively. For this purpose, calorimeters are used to determine the heat flux and enthalpy flows in a plasma atmosphere that is created by an arcjet. Heat fluxes on the order of 50 MW rrT and enthalpy flows of about 20 MJ kg have to be measured at temperatures around 10 000 K. This can only be accomplished using transient techniques. To do so, the calorimeter, or the heat flux meter, is traversed through the arc and exposed to the plasma for only a fraction of a second, and the temperature increase of a thermocouple is measured. The calorimeter usually consists of a conical-shaped axisymmetric body of 25-50 mm diameter with a central bore in which a thermocouple is embedded close to the surface. Typical exposure times are 30 ms, resulting in temperature increases of several hundred kelvin. Traditionally, the measured temperature and the geometric parameters of the sensors are used to calculate the heat flux under the assumption of linear one-dimensional heat conduction. 

However, the same type of sensor can also be used in terrestric investigations, yet with a strongly reduced sensitivity. A Beckman-Monitor-System (type 123301 O2/T) was adapted to 100-mL vessels of a Calvet microcalorimeter. The electrode was placed high enough above the heat flux meter that no interference between the two signals could be observed. This system was - e.g. - applied during investigations of crabs [64] or snakes and lizards [38,65],... 

---