Study of air jet characteristics in air-conditioned rooms

The research deals with supply air jets in air-conditioned rooms which have different lengths. The study was conducted according to the numerical method using the Fluent software package. Discharge conditions are equal in all cases. As a result, dependences of main kinematic and geometric jet’s characteristics (width and jet range, longitudinal velocity profiles, axial velocity, average velocity of the back flow, flow rates in the cross sections of the direct and back flows, distribution of static pressure along the length of the jet) on longitudinal constraint parameter are determined (length of room). The calculation results are presented in the form of corrections to the characteristics of a free jet, taking into account the influence of longitudinal constraint. It is found that influence of Archimede’s buoyant force on the temperature distribution along the length of non-isothermal jet is inconsequential. Dependence of dimensionless temperature on the jet axis of dimensionless length of the room of the coordinate x is determined. The results can be used in calculation of airflow circulation in different purpose airconditioned rooms.


INTRODUCTION
Thermal comfort and air quality are the main goals of ventilation and air conditioning systems. Depending on the situation in the air-conditioned room, it is necessary to choose the appropriate air exchange scheme -the method of flows mixing or their driving out [1]. When choosing a mixing scheme, which has advantages for rooms with a high permissible velocity in the working area, air is conducted to the ceiling or floor, as well as horizontally, obliquely or vertically into the working area in the form of compact jets [2][3][4][5][6]. The flows created in the room using compact air supply are often non-isothermal and also carry various kinds of impurities. Thus, there is a task of calculating the velocity field, temperature and concentration in the jet and the room.
There are experimental and theoretical works which consider the characteristics of jets flowing into a room [7][8][9][10][11][12]. In recent years [10][11][12], when studying isothermal jets, it was found that the characteristics of the jet (width and jet range, longitudinal velocity profiles, axial velocity, average velocity of the back flow, flow rates in the cross sections of the direct and back flows, distribution of static pressure along the length of the jet) depend on the location and size of the holes through which air is supplied to and removed from the room, as well as on the size of the room itself.
Non-isothermal jets are studied much less, although it is obvious that the difference in the air densities of the flowing jet and the air-conditioned room can lead to significant deformation of the jet. The effect of non-isothermality on the distribution of jets in the rooms ventilated through active chilled beams was considered in the works [13,14]. In the works [15,16], the characteristics of a nonisothermal jet in the cubage of large rooms are numerically studied by CFD modeling. However, there is no study of the jets in a confined space, when the dimensions of the jet and the room are commensurable, and confined air jets develop according to a dead end or flow pattern. Therefore, so far, we do not have satisfactory answers to such fundamentally important questions as energy-efficient air distribution in rooms, patterns of dynamic and thermal jets circulating in limited areas, etc.

Materials and methods
This article numerically determines the characteristics of a plane nonisothermal jet developing according to a dead-end scheme. Fig. 1 illustrates the pattern of the flow at the dead end, one end of which is muffled and the other is opened. The study was conducted according to the numerical method using the Fluent software package on room models (dead ends) of the same height , but different length: "long dead end" ; "medium" dead end ; "short" dead end . The conditions for the jet discharge are the same in all cases: the initial velocity ; initial temperature 0 313 Т К ; width of the inflow slot ; initial flow rate per unit length of the slot .  , , The dead end height is taken as a linear scale, i.e. ; These are the necessary dependences for the free jet [3,4]: Eq.  It is important to note that the jet range in the "long" dead end, identified by Fig. 2 and Fig. 3 is not the same. The difference (approximately twofold) is due to the fact that the axial velocity at 4,5 4,8 x ! is close to zero but not equal to zero. For practical purposes, it is more convenient to define the range as the distance at which the axial velocity reaches a certain low specified value.   ). Then, the jet width decreases actively, which is explained by rarefaction in this area (see Fig. 6b). At 2 x | rarefaction on the jet axis reaches a maximum in the same section 1 x y k . This is followed by a turnaround area and the width of the direct flow decreases up to a minimum value at д x x . The static pressure is maximum here. Fig. 7 shows the results of the calculation of the air flow in the direct stream. The ejection effect leads to the fact that at small distances the flow rate in   We can state the qualitative correspondence of the profiles calculated numerically and according to Tarnopolsky formula [8] at 0,5 x . At large х the profiles are asymmetric as they are in accordance with Figure 5.2. Fig. 8 shows a change curve of the relative velocity in the direct (8a) and back flows (8b). As for axial velocity, before the start of the turnaround area ( 2 х | ), the average velocity increases to the maximum value and then drops to 0, indicating the range of the jet. Comparing the results obtained with the data given in the work [8], it can be said that the characteristics of the flow of the investigated jet are practically the same. This is explained by the fact that the jet is slightly non-isothermal and the longitudinal constraint factor affects only when д l x d . There is a dependence of the dimensionless temperature on the coordinate x suitable for air-conditioned rooms of any length.

INSTRUMENTATION SYMBOLS
p pressure (kPa) u velocity (m/s) T temperature (K) L rate (m 3 /s)