An experimental investigation on thermal striping: Mixing phenomena of a vertical non-buoyant jet with two adjacent buoyant jets as measured by ultrasound Doppler velocimetry

doi:10.1016/S0029-5493(99)00006-0

Nuclear Engineering and Design

Volume 188, Issue 1, 1 April 1999, Pages 49-73

https://doi.org/10.1016/S0029-5493(99)00006-0 Get rights and content

Abstract

An experimental investigation on the thermal mixing phenomena of three quasi-planar vertical jets, with the central jet at a lower relative temperature than the two adjacent jets, was conducted. The central jet was unheated (‘cold’), while the two adjacent jets were heated (‘hot’). The temperature difference and velocity ratio between the heated (h) and unheated (c) jets were, ΔT_hc=5°C, 10°C and r=V_cold,exit/V_hot,exit=1.0 (isovelocity), 0.7, 0.5 (non-isovelocity) respectively. The typical Reynolds number was Re_D=1.8×10⁴, where D is the hydraulic diameter of the exit nozzle. Velocity measurement of a reference single-jet and triple-jet arrangement were taken by ultrasound Doppler velocimetry (UDV) while temperature data were taken by a vertically traversed thermocouple array. Our UDV data revealed that, beyond the exit region, our single-jet data behaved in the classic manner. In contrast, the triple-jet exhibited, for example, up to 20 times the root-mean-square velocity values of the single-jet, especially in the regions in-between the cold and hot jets. In particular, for the isovelocity case (V_exit=0.5 m/s) with ΔT_hc=5°C, we found that the convective mixing predominantly takes place at axial distances, z/D=2.0–4.5, over a spanwise width, x/D∼|2.25|, centered about the cold jet. An estimate of the turbulent heat flux distribution semi-quantitatively substantiated our results. As for the non-isovelocity case, temperature data showed a localized asymmetry that subsequently delayed the onset of mixing. Convective mixing however, did occur and yielded higher post-mixing temperatures in comparison to the isovelocity case.

Introduction

Thermal striping refers to random thermal cycling of reactor structures and components as a result of fluid–structure interaction; that is, striping is likely a description of the cold and hot (thermal) stripes appearing as plumes and jets, that a solid boundary must withstand due to preferential or inefficient mixing of coolant flowing through and exiting the reactor core. The net result of striping is undesirable since thermal fatigue of materials can lead to structural and material failure. Thermal striping as a phenomenological problem in LMFBRs was already recognized in the early 1980s by Wood (1980) and Brunings (1982) and has subsequently been considered by Betts et al. (1983), Moriya et al. (1991), Muramatsu (1994) and Tokuhiro (1996).

We note here that, although the phenomena taken as a whole involve fluid–structure interaction, the analytical and experimental efforts have traditionally been divided into separate structural and thermal-hydraulic investigations. In the present work, we focus strictly on the thermal-hydraulic aspects; that is, mainly the convective mixing of a multiple number of jets at different temperatures and average exit velocities. In the past, investigations on jets have encompassed the single-jet, which has most extensively been studied, to two jets flowing side-by-side, at a relative angle or co-axially and with a relative velocity (and/or temperature) with respect to each other. In fact, in the LMFBR sector, co-axial jets of sodium have been investigated by Tenchine and Nam (1987) while Tenchine and Moro (1995) compared the results of sodium and air jet experiments. Investigations of more than two jets seem to be rare. Thus besides its relevance to LMFBR thermal-hydraulics, a study of a multiple number of vertical jets at either the same or different densities (temperatures), may be of general interest to the heat transfer community.

In the present study, we carried out water-based experiments in a test facility simulating the mixing of one centrally located, unheated jet sandwiched by two adjacent jets either buoyant (at higher temperature) and/or at different exit velocity relative to the central jet. The three-jet arrangement is a simplified simulation of hot and cold flow channels in a LMFBR core. An understanding of thermal striping or rather the convective mixing is one of the key issues in the safe design of the LMFBR. Experimentally, one objective of the study was to demonstrate the applicability of the ultrasound velocity profile (UVP) monitor for velocity measurements. By applicability we mean velocity measurements in the flow field of relevance. Subsequently, we first obtained and evaluated the hydrodynamic information concerning the nature of mixing between thermally-stratified jets. Then with the addition of temperature data we were able to assess the thermal-hydraulics of mixing process.

Section snippets

Experimental facility

Fig. 1 shows the experimental loop including the test section. Except for the test section, the rest of the facility functions as a support system shared by two other experiments. The facility thus consists of the thermal striping test section set within a larger rectangular tank, a loop heater/exchanger for supplying hot water, a head tank in order to control the water level, a filter to extract contaminants within the loop, an air-to-loop heat exchanger for supplying cold or cooled water back

Photographs and video images

We first present in Fig. 4(a) and (b) digitized image sequences of respectively, the single- and triple-jets extracted from video as a qualitative introduction. The images have been taken with laser-sheet (argon laser) illumination from the right side with Rhodamine dye added to water. An horizontal line tracing the laser sheet beam is clearly visible on the top surface of the four blocks. Fig. 5 depicts a typical frame-by-frame sequence of the triple-jet at different average exit velocity and

Conclusions

An experiment investigating the thermal-hydraulic mixing of three quasi-planar, vertically flowing (water) jets was conducted. In the experiment the central jet was unheated (cold) and therefore non-buoyant, while the two adjacent jets were heated (hot) and therefore buoyant. The three jets flowed into a large volume of water initially at the central jet’s temperature. The ratio of the cold-to-hot jets’ average exit velocities (and flowrate) was equal to r=V_cold,exit/V_hot,exit=1.0

Nomenclature

D	hydraulic diameter of the inlet channel (mm)
ML	measurement length; pertaining to ultrasonic beam path
Q_turb	turbulent heat flux, turbulent heat flux at exit, =ρC_p u’_RMS T’_RMS
r	cold-to-hot jet average velocity ratio at exit, =V_cold,exit/V_hot,exit
R, L	from right, from left
Re	Reynolds number of inlet channel, =(UD/ v) or (Uz/ v)
SD	standard deviation of average velocity
TDX	transducer
T	temperature (°C)
T_h(ot)	temperature of ‘hot’ jet (°C)
T_c(old)	temperature of the ‘cold’ jet (°C)
ΔT_hc	temperature difference

Acknowledgements

The first author would like to thank PNC for his appointment as PNC International Fellow. The authors also acknowledge the efforts of Mr Miyakoshi, Mr Itoh and Mr Onuma who initiated and conducted the experimental measurements, maintained the data and prepared the UVP and temperature data which were analyzed in order to prepare this report.

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An experimental investigation on thermal striping: Mixing phenomena of a vertical non-buoyant jet with two adjacent buoyant jets as measured by ultrasound Doppler velocimetry

Abstract

Introduction

Section snippets

Experimental facility

Photographs and video images

Conclusions

Nomenclature

Acknowledgements

Int. J. Heat Fluid Flow

Nucl. Eng. Design

Thermal striping in liquid metal cooled fast breeder reactors

Modeling turbulent jets with variable density