Heat transfer of flooded impact jets

This paper presents the results of experimental studies on providing heat to the inner surface of the shell plating of the ship through the use of impact jets. In this work, we carried out (by taking into account the existing design features) a modeling and a study of the flow and heat transfer of flooded jets that flow over a flat surface and then spread radially. The visual study revealed different patterns of radial flow of the wall jet. Critical Reynolds criteria and relevant summarizing similarity equations describing heat transfer are determined.


Introduction
The problem of creating of highly effective environmentally friendly marine engine cooling systems required the development of an efficient method of heat transfer through the shell plating (Fig. 1). For the case described water is almost still on the inner side of the shell plating. Heat transfer from the cooled water to the inner surface of shell plating is carried out with gravitational convection. It is well known that the values of heat transfer coefficients in this case are rather small. The use of impact jets allows significantly increasing he heat transfer using relatively simple means. Impact jets are widely used in practice. They simplify surface heating or cooling processes. At the same time, they allow achieving high enough values of heat transfer coefficients. At the nozzle's outlet, an axisymmetric jet is formed which is transformed into a diverging radial wall jet after a collision with the surface. At the same time, the following factors have an impact on these processes: design features of the nozzle-surface system, Reynolds criterion, and other factors [1,2,3,4].
A distinctive feature of the shell plating is the presence of power components on the inner side. These power components are usually positioned perpendicularly to the surface of the shell plating (Fig. 1). The liquid is supplied into the space between the elements of the frame. It is clear that such a structure will affect the impact jet flow pattern and the heat transfer process. Therefore, it is required to determine the characteristics of the processes and the corresponding dependencies describing heat transfer.
To obtain this information experimental studies have been carried out.

Experiment
For more complete understanding of the underlying processes we carried out combined visual and thermal engineering studies. For this purpose corresponding experimental plants have been created. The used model is a rectangular container with a transparent wall with dimensions 0.25x0.25m. This wall simulated the shell plating of a ship. On the inner side, perpendicularly to the wall, in the center, the inlet nozzle was installed. There was a possibility to vary the distance from the nozzle to the wall. The container has been completely filled with water, which created conditions for a formation of a submerged jet. The model provided control of the fluid flow through the nozzle. It allowed achieving the maximum speed of water in the nozzle of 1.8m/sес, which corresponded to the value of the Reynolds criterion of . For the flow visualization purposes we added water based darkening liquid, aluminum paint, and a surface-active agent. We carried out observations from outside the transparent wall, which was additionally illuminated with a direct light.
Thermo technical studies were carried out on a specially designed experimental model (Fig. 2). It consisted of two parts: space 1 (hot cooled water) and space 2 (cold cooling water). The dimensions of the former were × and thickness of 5mm, through which the transfer of heat was carried out. The supply of the hot water to the center of wall 3 was carried out through an insulated pipe 4 with a screwed end. To this screwed end 4, various attachments (such as a nozzle) could be fixed. There also existed an opportunity to change the distance h from the end of the nozzle to wall 3. The space 1 has been completely filled with water. All lateral surfaces of the experimental model have been carefully insulated.
We measured the temperature of wall 3 on nozzle's side using seven сhromel-сopel thermocouples with the diameter of thermo electrodes of 0.15mm. We measured the thermo EMF using a potentiometer. In our model, we also measured the temperature of inlet and outlet of hot and cold water.
The experimental model was connected to the thermal hydraulic stand. There was a possibility to control the flow rate of hot water. Maximum value of flow rate reached 11m 3 /h. There also was a coolant flow measuring system. System electric heater allowed setting different power values. In the course of experiments, water temperature reached 80…85°C, which corresponds to the maximum possible temperature level in real conditions.
The pressure drop on the test device was determined using a model pressure gauge. At low liquid flow rates (low pressure drops) the measurements were carried out using a hydro-differential pressure gauge. and outlet pipes, respectively; 6 and 7: cooling water inlet and outlet pipes, respectively.

Results and discussion
The results of the accomplished visual studies at different values of Reynolds critera are shown in Fig.3. The liquid that leaves the nozzle hits the surface and then spreads in radial directions, forming a radial wall jet.
It is clearly seen that as Re changes, the pattern of the flow along the surface changes accordingly. For instance, when , multiple developed turbulent swirls are observed throughout the area. The intensity of these turbulent swirls is maximal in the center and decreases as the distance from the center increases. . In the course of direct observation of the processes taking place in the depth, at a distance from the wall, secondary flows moving toward the nozzle have been detected. This was caused by suction of fluid from the surrounding space, which confirms the existing understanding of fluid movement in the area of the outlet nozzle edge [4,5,6].
Thermo technical studies were conducted using nozzles with an inside diameter of 18mm and 38mm. The distance h from the nozzle to the surface was set to 5mm, 12mm, and 20mm. The dependence of the average (across the surface) heat transfer coefficient α on the rate of fluid flow W through the nozzle at different diameters is shown in Fig. 4. Increasing W leads to increasing of α values. However, no interaction between heat transfer coefficient and diameter d of the nozzle has been revealed. Fig. 5   The results of experimental data summarizing are shown in Figure 6. (1) .    . In laminar and turbulent flows, these values equal to, 0.45 and 0.4 respectively. The results for the laminar flow should be compared with the results of [6,7,8]. As the range of the Reynolds criteria offsets towards lower values, the slope of the line decreases. In the case of [9], the slope becomes almost equal to that observed in the present work. A certain exception in this regard is the results of [10], where the slope is large enough, but it does not exceed the value observed for the transition area. In this case, the design of the inlet nozzle or a high turbulence of the supplying flow could impact the results [5].

Сonclusions
Visual studies conducted in this work revealed different wall jet flow patterns. This allowed determining the heat transfer summarized dependencies that correspond to each pattern. Eventually, using rather simple methods the initial problem of increase of heat transfer through the shell plating of the ship is solved.