Experimental study of transient processes in the elements of climatic systems

. Transition processes for different modes of functioning of elements of the climate system model are experimentally investigated and analyzed. The following elements have been selected as the objects of experimental research: the air heater and the fan as the main elements that are part of each climate system. The system under study consisted of the following sequential elements: air heater-fan-room. The relative excess heat is proposed as a dimensionless parameter in the analysis and normalization of experimental data. First, the relative excess heat takes into account changes in temperature, moisture content and flow rate of humid air, and secondly, allows identifying similarities in the processes of change over time of the thermodynamic state of the air when passing through the climate system elements for different operating modes of the fan and air heater. Analysis of the evolution of relative excess heat during transient changes in the parameters of the flow of humid air, as in case of abrupt changes in performance of a heater but with a fixed performance of a fan and in case of abrupt changes in performance of a fan, but with a fixed power of the air heater. The regularities of the evolution of relative excess heat in the flow of moist air and its most important qualitative types when passing through the elements of the model climate system are shown. It was found that the types of behavior of relative excess heat depend only on the state in which the elements of the system are located during the transition process, and not on the element itself as a physical object.


Introduction
Providing thermal conditions in buildings for various purposes requires stabilization of the specified parameters of the microclimate at a certain level. This requires, on the one hand, certain material and energy resources. On the other hand, this is connected with the concept and development of climate systems, which should compensate for the constantly changing external and internal disturbances of the heat and moisture flows, etc.
External and internal disturbances of microclimate parameters can be periodic (for example, daily, seasonal) or random. This determines the non-stationary thermodynamic mode of climatic systems.
Because of the level of requirements to the quality of the microclimate, the climate system can be functionally quite simple, for example, convection heating and quite complex, for example, the air conditioning system in combination with radiant systems for heating and cooling [1,2]. No matter how complex the climate system may be, it consists of the main elements: heat and mass transfer devices (heat exchangers, humidifiers and dehumidifiers), pressure lifting devices (pumps, fans, compressors) [2]. But at any level of complexity of climate systems, however, the task is to study transients of changes in the parameters of the state of a stream of humid air as it passes through the elements of the climate system, that is, to study the response of a stream to an external disturbance.
The importance of such research is based primarily on the attention of specialists in the design and development of control systems and automated microclimate quality control. In most cases, thermodynamic and transient processes of heat transfer have been investigated theoretically only [3][4][5][6][7][8][9][10]. In our opinion, the reason is quite obvious. Experimental studies are quite expensive and require a significant amount of time and experimental installation corresponding to real climate systems. In particular, the processes occurring in real conditions should be adequately reproduced.

Experimental setup and measurement techniques
Experimental studies were carried out on the installation of standard elements that are used in the design of ventilation and air conditioning systems. The installation was equipped with a computer-controlled informationmeasuring system (Fig. 1).
The information-measuring system recorded the temperature at the inlet and outlet of each element of the heater-fan-room system: t1 and t2 for the air heater, t2 and t3 for the fan and t3 and t4 for the room. The flow rate (u), the static pressure by the fan (ΔP), and the relative humidity in the flow (φ) were also recorded. The measured values of the sensors were recorded equidistant in one second in the control sections. Power control of the system elements was automatic: the air heater was equipped with a triac power regulator AW, and the fan -frequency regulator AF.
The transition process was started abruptly, by changing the performance of one of the elements of the air heater-fan system, with a fixed air flow of the other. The value of the surge in the air flow of elements varied by a certain amount in fractions of the maximum power of the element and ranged from 0.1 to 0.5 with a step of 0.1.

Measurement result
To generalize and analyze the experimental data of transients in climatic systems, a parameter is required that takes into account both the change in the flow temperature and its velocity. Therefore, we have proposed a relative excess heat, calculated as follows. For the apparent heat generated by the air heater (2) and assimilated by the room air is the heat released by the element into the current , the initial  = 0, and the final *  =500s times.
Similarly, the calculated relative excess flow Experimental dependences of the parameters obtained during the processing of experimental data, namely: changes in the flow temperature, velocity and relative values of heat and flow (1) -(4) with a sudden increase in the power of the heater (flow heating) are shown in Fig. 2 and with an abrupt increase in the fan performance (acceleration of the flow) in Fig.3. Consider the behavior of the flow parameters in the process of transition from the initial moment of time to the final. When the flow is heated by the air heater, t2 will increase, Fig. 2. The experimental data correspond to a constant fan capacity of 0.9 (63 Hz) and an increase in the thermal power of the air heater from 0.5 (6 kW) to 0.8 (9.6 kW). In some series, including this article (Fig.  2), it was found the excess temperature t2 over t3 in the time interval from 75 to 300 s. This is due to the high heat capacity of the "body" of the fan, the heat exchange between it and the air that flows it. The higher the fan performance, the more intense the heat exchange between the fan and the flow and the smaller the difference between t2 and t3. At high flow rates, this difference may be negative, as shown in Fig. 2. Temperature t4 characterizes the flow to be removed from the room serviced by the system. It does not a monotonously curve. When heating the stream (τ ~ 75 s), a small minimum is observed. This is due to the volume of air in the room, the value of the fan performance and, therefore, depends on the multiplicity of air exchange. The higher this value, the less the assimilating capacity of the air in the room affects the microclimate parameters.
The evolution in time of the relative excess heat r Q released by the air heater, absorbed by the fan and assimilated by the air of the room when the flow is heated is shown in the lower part of Fig. 2. When heating the flow, the heat released by the air heater increases r h Q . On the contrary, the amount of relative excess heat r f Q absorbed by the fan during the heating of the flow increases at the initial stage and reaches a minimum, and then decreases and stabilizes. The extremum of the evolution of the relative excess heat absorbed by the fan is due to the different rates of temperature change before and after the fan (t2 and t3) during the transition process.
The evolution of the relative excess heat assimilated by the room r r Q air goes in antiphase with a shift Δ to the heat generated by the air heater. The figure 2 shows fan operation modes at Δ ≈ 10 s. With decreasing fan performance, the delay Δ increases. For example, for fan operation mode 0.4 (28 Hz) Δ ≈ 16s. What is completely permissible to associate with the rate of air exchange, i.e. with decreasing multiplicity of air exchange, the delay Δ grows.
The transition process during flow cooling, i.e. with a sharp decrease in the heating capacity of the air heater, is similar to heating, with the only difference that the processes of change  Fig. 3 shows the data on acceleration of the flow, obtained with a fixed heating capacity of the heater 0.3 (3.6 kW) and fan performance from 0.5 (35 Hz) to 0.8 (56 Hz). When disperse, the t2 goes down. In this case, t3 decreases somewhat slower than t2, which is associated with an increase in productivity and is correlated with a significant increase in the fan's own heat emission. In addition, the larger the sourge of the flow velocity u, the greater the increase of the temperature difference between t2 and t3. Temperature t4 characterizes the flow removed from the room and it does not monotonously curve. When disperse t4 first increases slightly, and then falls monotonously. The presence of such a phenomenon is mainly associated with the volume of air in the room and the magnitude of the jump in the performance of the fan. The greater the sourge and, accordingly, the greater the increase in the rate of air exchange, the less the assimilation capacity of the air in the room affects the parameters of the microclimate. During the acceleration of the flow, the relative excess heat assimilated by the room air

Results And Discussions
In our studies, the transient process was considered as a response of the flow state parameters to the discontinuous influence of an element of arbitrary size from the moment of application of this effect to a certain steady state.
The evolution of the relative excess heat of the flow passing through the air heater Q shows that their qualitative representation is provided only by the state of the elements of the climate system in which they are during the transition process. As can be seen from Figs 2 and 3, three basic states of the system element can be distinguished: active, reactive and passive.
The active state is characteristic of the object of regulation with an internal heat source (in Fig. 1 this is an air heater, in Fig. 2 a fan).
Reactive is for an unregulated object, but also with an internal heat source reacting to an external perturbation of the flow by the active element (On Fig.1 is a fan and on Fig. 2 is air heater). And the passive state for the object of control (in both Figures 2 and 3 is a room) that does not have its own sources of heat or cold (except for rooms with enclosing structures with built-in radiant heating-cooling panels), where the microclimate is formed by internal and external sources and heat sinks [11][12][13][14][15].
It should be clarified that objects with internal sources of heat mean objects in which the internal source of heat is an integral part, without which the object cannot perform its functions. For the premises, internal and external sources and flows of heat (heat from people, technological and other climatic equipment, heat losses and heat gain through the enclosing structures, etc.) do not affect the main functions of the enclosing structures, namely protection from direct atmospheric influences.
Let us consider a more detailed effect of the system element on the state of flow parameters and the evolution of relative excess heat. Turning to the active state, the element generates more heat at the end of the transition process during heating or acceleration of the flow than at the beginning. This is indicated by the direction r h Q for +1 when heated (Fig. 2) and r f Q for +1 at acceleration of a stream (Fig. 3). This is a consequence of growth ) Fig.3), which determines the positive sign of relative excess heat at the end of the transition process for the active element. The flow rate is always positive, when it is heated it is constant, and during acceleration it quickly grows and stabilizes. In the reactive state, an element with its own source of heat of a certain mass directly influences the evolution of relative excess heat. Moreover, the evolution of the reactive element r f Q (Fig.2) develops in the antiphase evolution of the active element, and r f Q (Fig.3) -in phase. This is explained by the fact that during the heating of the flow decreases rapidly (Fig. 2) and, therefore, It should also be noted that the elements in the reactive state are characterized by the appearance of the evolution extremum: for the fan r f Q (Fig.2) and air heaters r h Q (Fig.3). The presence of extrema of evolution is a consequence of different rates of temperature change at the inlet and outlet of the element.
For example, the extremum of evolution of r f Q (Fig. 2) caused by a faster growth of t2 than t3 at the beginning of the transition process, which leads r f Q to -1. This behavior is demonstrated by evolution r h Q (Fig.3). There is also a significant difference in the rate of temperature change: t2 decreases faster than t1, with a sharp increase in the speed u. If the temperature changes rates of t1 and t2 were the same, then ) (  (Fig.2), tending to -1, and during acceleration of the flow behaves in phase to the active element, tending to +1 (Fig.3). This is explained by the fact that the room, having an internal space with a certain mass of air, is a heat-inertial element of the system under study. When the flow is heated by an air heater (Fig.2) the room air absorbs some of the heat and t4 grows, and during acceleration of the flow fan (Fig.3) gives and t4 decreases. Thus, when heating the flow increases and decreases during acceleration.

Conclusions
The presence of transients in the elements of climatic systems with variable parameters (temperature and flow) of the coolant (air, water, etc.) is characteristic and requires a universal criterion for their analysis, classification and use in the optimization of control processes. As such, the criterion in this paper is the relative excess heat r Q . This allowed not only to generalize and normalize the obtained experimental data, but also to reveal for the first time the regularities of the evolution of the relative excess heat and the qualitative form of functional relationships between the initial and final flow parameters. Qualitative identification of the experimental data allows us to further functionally describe the evolution of relative excess heat for a wide class of climatic systems. Thus, the functional and parametric identification of the evolution of the relative excess heat r Q of the flow passing through the elements of the climate system will make it possible to determine the current parameters of the flow state during the transition process at known initial values. Therefore, it is possible to solve the direct problem, i.e. to predict the reaction of the flow to the disturbance, both for individual elements of the climate system and the system as a whole, and for the room. Since the reaction to the possible disturbance of the moist air flow by the elements of the air heaterfan-room system is predictable in almost all time frames, a real tool for monitoring and controlling the microclimate in the room appears.