J.W. Mitchell

Behavioral implications of mechanistic ecology

SummaryMechanistic principles from engineering, meteorology, and soil physics are integrated with ecology and physiology to develop models for prediction of animal behavior. The Mojave Desert biome and the desert iguana are used to illustrate these principles.A transient energy balance model for animals in an outdoor environment is presented. The concepts and relationships have been tested in a wind tunnel, in a simulated desert, and in the field. The animal model requires anatomical information and knowledge of the thermoregulatory responses of the animal. The micrometeorological model requires only basic meteorological parameters and two soil physical properties as inputs. Tests of the model in the field show agreement between predicted and measured temperatures above and below the surface of about 2 to 3°C.The animal and micrometeorological models are combined to predict daily and seasonal activity patterns, available times for predator-prey interaction, and daily, seasonal and annual requirements for food and water. It is shown that food, water and the thermal environment can limit animal activity, and furthermore, the controlling limit changes with season. Actual observations of activity patterns and our predictions show close agreement, in many cases, and pose intriguing questions in those situations where agreement does not exist. This type of modeling can be used to further study predator-prey interactions, to study how changes in the environment might affect animal behavior, and to answer other important ecological and physiological questions.

Heat transfer from spheres and other animal forms

A general predictive relation for the convection heat transfer from animal forms is developed. This relation is based on the convection equation for a sphere, and employs a simple, unique characteristic dimension to represent the animal which is the cube root of the animal volume. The accuracy of this relation is established through comparison with available convection results from animal shapes ranging in size and shape from spiders to cows. This relation allows an extrapolation to animal shapes for which data are not available. Results are also presented for the enhancement of convection heat transfer due to natural turbulence. A procedure is outlined for estimating the convecture heat loss from an animal in the natural outdoor environment.

Performance of combined solar-heat pump systems

Abstract A comparative study of the performance of combined solar heat pump systems for residential space and domestic hot water heating has been undertaken. Simulations have been made with TRNSYS[1] of three basic combined configurations, as well as conventional solar and conventional heat pump systems, in two different climates, Madison, Wisconsin, and Albuquerque, New Mexico. The three combined systems are the series system in which the solar storage is used as the source for the heat pump, the parallel system in which ambient air is used as the source for the heat pump, and the dual source system in which the storage or ambient is used as the source depending on which source yields the lowest work input. The influence of collector area, number of glazings, main storage volume to collector area ratio, and heat pump coefficient of performance were determined. The results indicate that the parallel combined system is probably the most practical solar-heat pump configuration. The thermal performance at a given collector area is consistently superior to both the series or the dual source systems over the heating season. Costs and the extent to which summer cooling is a requirement determine the relative merit of the conventional heat pump, conventional heat pump, conventional solar, and parallel systems.

Open-Cycle Desiccant Air Conditioning as an Alternative to Vapor Compression Cooling in Residential Applications

The performance of open-cycle desiccant air conditioners for residential applications is evaluated. The performance of these systems is compared to that of vapor compression air conditioners on the basis of primary energy use and cost. Systems with improved dehumidifiers can achieve seasonal COPs on the order of 1.1. These systems, when coupled with a solar energy system to supply regeneration energy, are significantly better than conventional air conditioners on a primary energy basis, but are not presently cost-competitive.

Design theory for rotary heat and mass exchangers—I. Wave analysis of rotary heat and mass exchangers with infinite transfer coefficients

Abstract A theory is presented for modeling rotary heat and mass exchangers with infinite transfer coefficients. The continuity and energy conservation equations for one-dimensional transient flow are established and analyzed. Solutions to the equations are obtained by the method of characteristics and the shock wave method. Both methods provide a set of analytical equations that allow performance prediction of the heat and mass exchanger with infinite transfer coefficients for any inlet and operating conditions. A regenerator-operating chart is introduced which shows the fundamental modes of operation for rotary heat and mass exchangers. Part II presents correlations for the effectiveness of rotary dehumidifiers with finite transfer coefficients.

Transfer Functions for Efficient Calculation of Multidimensional Transient Heat Transfer

Finite difference or finite element methods reduce transient multidimensional heat transfer problems into a set of first-order differential equations when thermal physical properties are time invariant and the heat transfer processes are linear. This paper presents a method for determining the exact solution to a set of first-order differential equations when the inputs are modeled by a continuous, piecewise linear curve. For long-time solutions, the method presented is more efficient than Euler, Crank-Nicolson, or other classical techniques.

Development of a concept inventory for fluid mechanics

Concept inventories are assessment tools designed to determine the degree to which students understand the concepts of a subject and to identify the misconceptions that students hold. The results of a concept inventory can be used to change the methods of instruction to overcome student misconceptions. A cooperative effort between mechanical engineering faculty at the Universities of Wisconsin-Madison and Illinois, Champaign-Urbana has been directed toward development of a fluid mechanics concept inventory (FMCI). Fluid mechanics typically follows thermodynamics in the sequence of courses in thermal sciences, involves both the mechanics and dynamics of fluids, and builds on basic physics and Newtonian mechanics. This paper describes the process used for development of the FMCI, the details of how we determined the content, and examples of actual content of the instrument itself.

Design theory for rotary heat and mass exchangers—II. Effective n ess-number-of-transferunits method for rotary heat and mass exchangers

Abstract Analytical performance correlations for rotary heat and mass exchangers with infinite transfer coefficients have been presented in part I. A finite-difference model for performance prediction of rotary dehumidifiers with finite transfer coefficients is used in combination with the ideal dehumidifier model to establish effectiveness correlations. Correlations for the humidity and enthalpy effectiveness for silica gel regenerators are given as functions of the dehumidifier number of transfer units (NTU). An Effectiveness-NTU model, incorporating the correlations for the effectiveness and ideal dehumidifier performance, allows rapid calculation of the dehumidifier performance. The correlations are valid for a wide range of operating conditions and account for the effect of unbalanced flow and high Lewis numbers.

Simulations of the performance of open cycle desiccant systems using solar energy

Abstract A feasibility study of open cycle air conditioning systems that use solid desiccants and solar energy has been performed. The two configurations evaluated are the ventilation mode, in which ambient air is continually introduced into the room, and the recirculation mode, in which room air is recirculated. Seasonal simulations for Miami, Florida, show that the auxiliary energy requirement for the ventilation mode is about one half that for the recirculation mode. The seasonal COP for the system using solar energy as the auxiliary is approximately 0.75. A conventional flat plate solar energy system of moderate size can provide a large fraction of the energy required to meet the sensible and latent loads of a typical house.

Seasonal storage of energy in solar heating

Abstract This paper focuses on several aspects of seasonal storage for space heating using water as the storage medium. The interrelationships between collector area, storage volume, and system performance are investigated using the transient simulation program TRNSYS. The situations for which seasonal storage is most promising are presented. Particular emphasis is placed upon design of seasonal storage systems. A design method is presented which is applicable for storage capacities ranging from a few days to seasonal storage. This design method, coupled with cost information, should be useful in assessing the economic viability of seasonal storage systems. Also investigated are the importance of the load heat exchanger size, tank insulation, collector slope, and year-to-year weather variations in system design.