Mathematical models of thermocontrol processes occurring in chemical reactors and climate control systems are considered. In the models under consideration, the temperature inside a domain is controlled by a thermostat acting on the boundary. The feedback is based on temperature measurements performed by thermal sensors inside the domain. The solvability of the corresponding nonlinear nonlocal problems and the periodicity of solutions are studied.
Introduction.We consider mathematical models of thermocontrol processes in which the temperature inside a domain is controlled by a thermostat acting on the boundary. The feedback is based on temperature measurements performed by thermal sensors inside the domain. The processes under consideration occur in chemical reactors and climate control systems.The temperature distribution in the domain obeys the heat equation, while the boundary condition involves a control function (the Dirichlet, the Neumann, and the Robin boundary conditions are considered). The control function satisfies an ordinary differential equation whose right-hand side is a nonlinear operator H depending on the "mean" temperature over the domain and taking the values 0 or 1. There are two temperature thresholds w 1 and w 2 (w 1 < w 2 ). If the "mean" temperature is less than or equal to w 1 , then H = 1 (the heating is switched on); if the "mean" temperature is greater than or equal to w 2 , then H = 0 (the heating is switched off); if the "mean" temperature is greater than w 1 and less than w 2 , then H takes the same value as "just before." Thus, the presence of the operator H provides the so-called hysteresis phenomenon, while the thermal sensors inside the domain cause nonlocal effects.Thermocontrol models similar to ours were originally proposed in [8,9]. By transforming the problem into an equivalent set-valued integro-differential equation, the existence of a solution was proved. The existence and uniqueness of solutions for two-phase Stefan problems with the Robin boundary condition involving a hysteresis control were studied in [4,6,12]. Some questions related to optimal control for heat conduction problems with hysteresis were considered, e.g., in [3].The question whether periodic solutions exist turns out to be much more difficult. In [7], a one-dimensional thermocontrol problem is considered under the assumption that the temperature of the thermostat changes by jumping. Thus, there is no coupling with an ordinary differential equation in this case. The existence of a periodic solution is proved. Its uniqueness in a class of the so-called two-phase periodic solu-