Hydrodynamics of Direct Contact Condensation Process in Desuperheaters

: Due to the global warming and environmental implications, the focus of household heating has shifted from fossil fuels towards environmentally friendly and renewable sources. Desuperheaters have been found an attractive option as a domestic provision for the warm water; they used steam induced direct contact condensation (DCC) as the major means to warm the water. The present study has been an attempt to investigate the hydrodynamics in the Desuperheater vessel experimentally, when the pressurized pulsating steam was injected into the vessel, where, the steam jet interacted co-currently with the slow-moving water. Visual flow visualization provided an overall flow picture that showed a circulation region when the pulsating steam was injected into the slow co-currently moving water and the peaked vorticity corresponded to the steam injection duration varying from 10-60 seconds. An array of 7 Hot Film Anemometers (HFA) was traversed axially and radially to determine the velocity fluctuations at 0 – 20 cm from the steam's nozzle exit. Vortical structures were obtained that corresponded to the entrainment of the steam with the surrounding cocurrently moving water. The circulation regions were thus exhibited in relation to the steam's injection durations as well as the downstream axial distance of 2 cm and 15 cm from the nozzle exit, which showed that the core local circulation at 2 cm, lost 75-79% of its circulation at 15 cm downstream of the nozzle exit.


INTRODUCTION
The demand for energy on a domestic level has increased over the years, forming a greater proportion of total energy demand. Several factors are responsible for this rise, including, population growth, growing economy and thus wealthier lifestyles causing an increase in the use of electronic devices and vehicles. Another facet of the issue is the increasing usage of energy resources like fossil fuels. Such fuels have a definite age and quantity, but their increasing usage devastates the global outlook by polluting the environment. Thus, attention has diverted to renewable energy resources with increased efforts to determine renewable sources as a replacement of fossil fuels. Household warm water contributes to a major share in energy consumption. e.g. based on descending order; it consumes 32% in South Africa (Nkomo, 2016), 29% in Mexico (Rosas-Flores et al., 2011), 27% in China (Mahmoudi et al., 2018), 25% in Australia (GOVERNMENT, 2010), 22% in Canada (Aguilar et al., 2005), 14% in Europe (Trends, n.d.) and 11% in USA (Allouhi et al., 2015).
There are many systems which exist to provide customized solutions suited to a household's warm water requirement. These systems depend upon the climate, the nature of the requirement, nature of energy resource, and the design of the system. Thus, the selection of a suitable energy system could reduce the cost of warm water production and help save on unnecessary usage of energy resources, whilst being environmentally friendly. There are numerous studies (Chow, 2010;Hepbasli and Kalinci, 2009;Jaisankar et al., 2011;Shukla et al., 2009) on methods being used to provide warm water to the households which heat-pumps, solar water heaters with phase change materials, and thermal/photovoltaic solar technology-based systems. All these studies are comprehensive reviews, 3 within which the usage of the desuperheaters have been elaborated in detail. Desuperheaters performed a cordial role in the provision of the warm water, irrespective of the sources from where they inducted steam. They were involved in the processes such as the direct contact condensation that became the central to warm the water. Yet myriad times, the desuperheater setup has been discussed in depth. Still, till the date, there wasn't any study to our knowledge that discussed the issue of the direct contact condensation (DCC) induced hydrodynamics within the mixing region in the desuperheater including the pulsating steam injection. The current study is an effort in this regard. In the current study, a detailed analysis has been provided for the hydrodynamic trends prevailed in the desuperheaters.
The present study focused on the effect of the short pulse high-pressure steam injection into the continuous very slowly flowing water, and thus, the overall effect of the pulse injection on the in-situ hydrodynamics was determined. The sequence of the events that occurred within the mixing chamber was characterized, and the flow structures like vorticities, right from the moment they came into being till the time they decayed, were described in detail. The details of the experimental setup and the sequence of performed experiments are given in the proceeding section.

EXPERIMENTAL SETUP
The experimental setup comprised of a desuperheater vessel, which is shown in  The steam was injected into the desuperheater through a nozzle attached to a vertical duct. The vertical duct was submerged in the vessel, and the nozzle was located at the axial centre of the desuperheater's vessel. The inner diameter of the vertical duct is 3 cm, the inner diameter, d1 of the nozzle is 2.5 cm, the throat diameter d2 is 1 cm and exit diameter, d3 is 1.5 cm. The length of the nozzle is 10 cm, and the diameter and length of the desuperheater vessel are 10 cm and 60 cm, respectively. The vessel was filled with subcooled water which moved with very lower velocity, 0.01cm/sec and the steam was 4 injected at the stable gauge pressure of 4 bars in pulsating mode. The steam's injection was controlled using a solenoid valve and an electronic control system (ECS) installed upon the main steam line (not shown) in Fig 1. Hot Film Anemometers (HFA) were used to measure velocity fluctuations associated with interfacial steam-water flow. A fixture was made to facilitate forward and backward movement of the seven HFA within the fluid medium in the vessel. Before performing the experiments, the steam's mean velocity was measured at the exit of the nozzle by using the pitot tube. The dynamic pressure measured at the front of the pitot tube can provide the axial steam's mean velocity ( ) at the location of the front face of the pitot tube, through the application of the Bernoulli's equation, expressed as, where ∆ is the pressure difference between total pressure at the mouth of the pitot and the static pressure and is the steam density. This velocity was used to non-dimensionalize the velocity values obtained from the HFA sensors. The ECS could also monitor the movement of this fixture in clockwise and anti-clockwise directions to control the movement of the HFA sensors along the axial axis through initiating forward and backward movement of the HFA sensors. The velocity fluctuations were measured along the axial (X-U) and radial (Y-V) directions. All the seven HFA sensors were used at same time such that the array traversed a distance of 20 cm along the axial, from the downstream towards the upstream by acquiring the data for 1 min at a single location along the axis. Then it traversed forward to a distance of 1cm, and again the measurements have been done. in this way the whole medium comprised of the mixture of steam and water has been scanned in a vertical plane from a distance of 20cm till the exit of the nozzle. Both of these velocity values as a function of the spatial distances along which these have been recorded gives us useful information related to the total circulation along the axial direction, and local circulation and the velocity distributions along the radial direction. The total circulation (Linden, 1973) was measured with the help of the velocity fluctuation along the x-axis and the area containing the axial and radial velocity fluctuations being measured provided the total circulation, of the vortical ring. Whereas, the local circulation in terms of the angular velocity distributions along the radial direction (Linden, 1973) was calculated by using the following relations, The experiments thus performed and the discussion on the acquired results has been presented in the following section.

RESULTS & DISCUSSION
In the current study, the steam was injected at 5 bars of gauge pressure into a desuperheating vessel in a pulsating mode. The flow hydrodynamics associated with the flow regimes evident in the vessel was investigated with special emphasis on the vortical structures and circulation flows generated within the concurrently slowly flowing water. The details of the accompanying results in this regard were given as follows. the steam has been injected inside the flowing water. It was larger than the corresponding length across which it prevailed and then decreased with the passage of time. It is interesting to state here that the growth rate of this circulation depends mainly on the entrainment of the surrounding water. However, the circulation motion diminished at a distance away from the exit of the nozzle, it is, therefore, the growth rate of the large vortical structure underwent through major changes as the core region of the circulation also varied. A possible reason of restricting the circulation between the steam and the co-currently flowing water could be the buoyancy influence of the steam, which destabilized the interface along with the momentum-driven entrainment that impacted the flow in a negative way, an observation that was convincingly supported by an earlier study (Maxworthy, 1974).

Circulation flow ring and Vortical Structures inside the flow regimes
The steam injected in this phase of the experiment for time duration varying from 10-60 seconds. The  Fig 2(b). The results have shown that for all the injection time durations (10-60sec), the dependence of the circulating vortical structure was very weak at the varying injection time. it was also confirmed that even the Reynolds number also didn't add any dominant effect on the diameter of the ring, this finding is in line with an earlier study (Liess and Didden, 1976a). The dependence of the length of the vortical circulation ring land diameter against the steam inlet pressure (5 bars), and injection time was also determined. It was estimated by first assuming that the velocity (U), which was measured on the average basis at the exit of the steam nozzle had a uniform distribution and the length (L) of the Steam induced vortical ( 0 ), during the time duration, t is given by the relation as follows (Kulkarny, 1977),

Effect of Injection time on the instabilities inside the flow regime
It should also be noted that when traversing the velocity sensors, flow instabilities were measured inside the flow which remained dominant till the time when the steam was injected into the water and the flow dissipated after the valve for steam injection was shut after steam's injection for a specified time period.
The instabilities being observed here, were analogous for the similar instances in the earlier studies (Krutzsch, 1939;Liess and Didden, 1976b;Maxworthy, 1972;Moore, 1974;Widnall, D O N A et al., 1974;Widnall, S. E. & Sullivan, 1973) with variations in steam injection duration or variations in Reynolds number. However, a few interesting trends of the instabilities' wave number can be seen in seconds. Afterwards, it shows an increasing trend initially; however, then suddenly a decreasing trend can be seen which emphasizes the dominant role by the dissipative effects in the current flow regime.
A straight forward reason for such a behaviour is due to the instabilities that have first shown an increasing trend, which is consistent with the dimensions of propagation of the circulation vortical ring which afterwards has been broken out, with the resultant profiles have shown gradually flattening profiles due to the dissipative character under the action of such dissipative forces. Although, the phenomena of the pulsating fluid injection into the water was described by a number of studies that included mostly the visualization studies, but here in the current manuscript we quantitatively discussed the effect of the pulsating steam injection into the water on the flow regimes involving interaction between steam and water. It should also be noted that we do accept the non-frozen nature of the date, but still on the average basis, the fluid regime has been characterized as much as we can. So far the accumulative results have been concerned which can be drawn on the basis of the results discussed till yet, It was found that the core region which was emerged, but remains attached to the nozzle exit and with the rise in the injection time, only a slight rise in the length of the core region was observed.
But still, the main core that was responsible for giving birth to the forward rolling large vortical structures, Yet such efforts didn't exactly predict the exact balance between the positive and negative vorticities but still the basic physical phenomenon that can be used as a basis for modelling such case, cannot be simply denied as a whole, since still in the region far from the injection point the viscous dissipation surely can become dominant over inertial forces to break down the large circulating structures.

Flow Hydrodynamics in the region far from the steam nozzle
It has been observed in a number of studies (Afrasyab et al., 2013;Khan, 2014;Khan et al., 2016bKhan et al., , 2016aKhan et al., , 2013 that the instabilities at the interface have lower amplitudes in the region near the nozzle exit which has been transformed into larger and larger amplitude instabilities as soon as the steam propagates into the water. Amplitude of such instabilities after some finite rise break down into the ringlike vortices, which causes the interaction between the fluids at the interface. A possible reason for such a behaviour can be seen in earlier studies [19] where an imbalance between the axial wave number and radial mode number was claimed (Widnall, D O N A et al., 1974). According to the observations quoted in the given studies, the breaking of the outer core did not take place uniformly, rather occurred in azimuthal direction with the formation of a net flow. It was further observed in another earlier study (Leibovich and Randall, 1972) which claimed the propagation of just a single wave along the central core and the wave had a finite amplitude and a large axial velocity. Due to the large axial flow velocity, the central core wave in the far-off region had a profound effect on the regime owing to the ring instability.

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The measurements at the far region from the nozzle exit, exhibited a non-frozen nature which depicted the highly fluctuating nature of the flow regime. The velocity fluctuations measured at the central core were having less amplitudes than the velocity amplitudes at the periphery of the circulating region. The main reason for this may be the higher axial mean velocities, and fluctuations in the velocity were marginal compared to the mean values. Also, the interface appeared to be turbulent at the far regions as well, and this was characterized due to the formation of the vortical structures owing to the entrainment of the surrounding water. The variations in the magnitudes of the vorticities in the far region were relatively large owing to the stronger interacting between the steam and the surrounding water.
To fully understand the effects imparted by the vortices and the turbulence at the far regions from the nozzle exit, the local circulations at two distances, i.e. 2 cm and 15 cm from the nozzle exit, were obtained (see Fig 4), which were when compared with the core local circulations at the distance of 2 cm, the local circulation was found to lose 75-79% of its circulation at a distance of 15 cm as shown in