NUMERICAL SIMULATION OF THE THERMAL CONTROL OF HEAT EXCHANGERS

The temperature control of outlet air by changing the water flow rate in a single-pass waterto-air cross-flow heat exchanger is investigated. The conservation laws are applied to finite control volumes and an implicit formulation is used for transient numerical solutions. Conjugate forced convection heat transfer from the tube is solved to calculate the temperatures of the air and water coming out of the heat exchanger. In the simulations the outlet air temperature is controlled by changing the water flow rate entering the heat exchanger using a proportional-integral (PI) controller. The range of controllability of the heat exchanger was studied first. Then disturbances in the form of step changes in the inlet air temperature, the air flow rate, and the set point temperature were separately introduced. The effects of the limiting-condition constraints and different control parameters on controlling the outlet air temperature are presented. The results show that the control behavior can be simulated numerically and that this control methodology is effective within limits.     

Reduction and non-linear controllability of symmetric distributed systems

This paper considers the ‘reduction’ problem for distributed control systems. In particular, we consider controllability of systems containing multiple instances of diffeomorphic components where the overall system dynamics is invariant with respect to a discrete group action. A subclass of such systems are systems with a set of identical components where the overall system dynamics are invariant with respect to physically interchanging these components. The main result is a proposition which shows that for an equivalence class of symmetric systems of this type, controllability of the entire class of systems can be determined by analysing the member of the equivalence class with the smallest state space. The reduction methods developed are illustrated by considering the controllability of a team of mobile robots and a platoon of underwater vehicles.     

Charge Photogeneration for a Series of Thiazolo‐Thiazole Donor Polymers Blended with the Fullerene Electron Acceptors PCBM and ICBA

Photoinduced charge separation in bulk heterojunction solar cells is studied using a series of thiazolo‐thiazole donor polymers that differ in their side groups (and bridging atoms) blended with two acceptor fullerenes, phenyl‐C₇₁‐butyric acid methyl ester (PC₇₁BM) and a fullerene indene‐C60 bisadduct (ICBA). Transient absorption spectroscopy is used to determine the yields and lifetimes of photogenerated charge carriers, complimented by cyclic voltammetry studies of materials energetics, wide angle X‐ray diffraction and transmission electron microscopy studies of neat and blend film crystallinity and photoluminescence quenching studies of polymer/fullerene phase segregation, and the correlation of these measurements with device photocurrents. Good correlation between the initial polaron yield and the energetic driving force driving charge separation, ΔE_CS is observed. All blend films exhibit a power law transient absorption decay phase assigned to non‐geminate recombination of dissociated charges; the amplitude of this power law decay phase shows excellent correlation with photocurrent density in the devices. Furthermore, for films of one (relatively amorphous) donor polymer blended with ICBA, we observe an additional 100 ns geminate recombination phase. The implications of the observations reported are discussed in terms of the role of materials' crystallinity in influencing charge dissociation in such devices, and thus materials design requirements for efficient solar cell function.     

Controllability of cross-flow heat exchangers

The operation of heat exchangers and other thermal equipment in the face of variable loads is usually controlled by manipulating inlet fluid temperatures or mass flow rates. This is a fundamental study of the basic issues regarding state and output controllability in such systems. A numerical method based on finite-differences is developed to approximate infinite-dimensional equations by finite-dimensional ones for the study of a conduction-convection system. The dynamics of a single-pass cross-flow heat exchanger with simultaneous advection, convection and conduction, in which water and air are the in- and over-tube fluids, respectively, is represented by a coupled set of partial differential equations. The numerical method is used to analyze the behavior of the heat exchanger equations. Using the water or air inlet temperature as the manipulated variable leads to a linear problem, and for the water flow rate it is non-linear. Controllability results for different choices of the manipulated variable are presented.     

Clinical impressions of acoustic reflex measures in an adult population

The implications of the use of sensitivity predictions from acoustic reflex (SPAR) measures are increasing through clinical application of the test. Jerger and associates have suggested the importance of SPAR in the examination of children. Our use of SPAR with a primarily adult population has supported other important applications of this procedure, including the the use of predictions in legal cases and in examining elderly or foreign-speaking patients who often respond poorly on subjective tests. Discussion and recommendations regarding such applications are based on a study of SPAR involving 681 ears.     

Controllability of kinematic control systems on stratifiedconfiguration spaces

This paper considers nonlinear kinematic controllability of a class of systems called stratified. Roughly speaking, such stratified systems have a configuration space which can be decomposed into sub-manifolds upon which the system has different sets of equations of motion. For such systems, considering the controllability is difficult because of the discontinuous form of the equations of motion. The main result in this paper is a controllability test, analogous to Chow's theorem, is based upon a construction involving distributions, and the extension thereof to robotic gaits     

Flow-based control of temperature in long ducts

Flow-based control of a thermal system with a long duct and heat loss to the environment is analyzed. A Proportional-Integral controller is used to regulate the duct outlet temperature by using the flow velocity as control input. The one-dimensional energy equation in Eulerian and Lagrangian forms are numerically solved. The non-linear dynamics can be represented by an integral equation in terms of the residence time which acts as a delay. A linear stability analysis leads to a characteristic transcendental equation which is examined for different orders of the residence time. Pontryagin’s theorem on the zeros of exponential polynomials is used to obtain stability maps as a function of system parameters. Numerical simulations are performed to verify the predictions, determine super- and sub-critical instabilities, and evaluate the amplitude and frequency of limit-cycle oscillations.     

Multi-agent compositional stability exploiting system symmetries

This paper considers nonlinear symmetric control systems. By exploiting the symmetric structure of the system, stability results are derived that are independent of the number of components in the system. This work contributes to the fields of research directed toward compositionality and composability of large-scale system in that a system can be “built-up” by adding components while maintaining system stability. The modeling framework developed in this paper is a generalization of many existing results which focus on interconnected systems with specific dynamics. The main utility of the stability result is one of scalability or compositionality. If the system is stable for a given number of components, under appropriate conditions stability is then guaranteed for a larger system composed of the same type of components which are interconnected in a manner consistent with the smaller system. The results are general and applicable to a wide class of problems. The examples in this paper focus on the formation control problems for multi-agent robotic systems.