Data on cloud water content are deduced from hot-wire power levels.
The figure shows a compact, rugged, simple sensor head that is part of an instrumentation system for making measurements to characterize the severity of aircraft-icing conditions and/or to perform research on cloud physics. The quantities that are calculated from measurement data acquired by this system and that are used to quantify the severity of icing conditions include sizes of cloud water drops, cloud liquid water content (LWC), cloud ice water content (IWC), and cloud total water content (TWC).
The sensor head is mounted on the outside of an aircraft, positioned and oriented to intercept the ambient airflow. The sensor head consists of an open housing that is heated in a controlled manner to keep it free of ice and that contains four hot-wire elements. The hot-wire sensing elements have different shapes and sizes and, therefore, exhibit different measurement efficiencies with respect to droplet size and water phase (liquid, frozen, or mixed). Three of the hot-wire sensing elements are oriented across the airflow so as to intercept incoming cloud water. For each of these elements, the LWC or TWC affects the power required to maintain a constant temperature in the presence of cloud water.
Each of these three elements is considered to be subject to two forms of heat loss. The first form consists primarily of convective loss attributable to the flow of air past the element. This form is sometimes termed the “dry” loss because it excludes the cooling effect of the impinging water. The second form of heat loss is the cooling effect of impinging water. When the element intercepts liquid cloud water, energy is lost from the element in heating the water from ambient temperature to the equilibrium temperature for evaporation, and further energy is lost as latent heat of vaporization. When the element intercepts cloud ice crystals, there is an additional loss consisting of the latent heat of fusion for melting the ice. In operation, each element is maintained at a temperature of 140 °C by a digital electronic feedback control subsystem. The power expended in maintaining this constant temperature is the measurement datum associated with the element.
The fourth hot-wire sensing element, denoted the reference element, is oriented along the direction of airflow so that it does not intercept cloud water but is still subject to convective cooling. Like the other three elements, the reference element is maintained at constant temperature. In the case of this element, the power needed to maintain the constant temperature is a measure of the dry heat loss and is thus termed the “dry” power. The cloud water content is estimated in a first-principles computation based on known relationships among the cloud water content, the hot-wire power levels, the dimensions of the sensor wires, ambient temperature, and true airspeed.
The measurements and computations needed to quantify cloud IWC (glaciation) and droplet size are more complex. It has long been known that the response of a hot-wire sensor to water droplets decreases with increasing droplet diameter. The response of a wider element is similar to that of a narrower element, except that the onset of the decrease occurs at a larger drop size. Although this droplet-size dependence is not fully theoretically understood, it is empirically known to be highly repeatable and to be useful as a means of inferring droplet diameter: Specifically, measurement data acquired under known conditions in a wind tunnel can be used to calibrate an instrumentation system like this one to enable determination of the median volume diameter of cloud water droplets, given the differences among the responses of the hotwire sensing elements.
This work was done by Lyle Lilie, Dan Bouley, and Chris Sivo of Science Engineering Associates, Inc. for Glenn Research Center.Inquiries concerning rights for the commercial use of this invention should be addressed to NASA Glenn Research Center, Innovative Partnerships Office, Attn: Steve Fedor, Mail Stop 4–8, 21000 Brookpark Road, Cleveland, Ohio 44135. Refer to LEW-18029-1.