Nuclear Engineering and Design 188 (1999) 49 – 73

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

A. Tokuhiro *, N. Kimura

Reactor Engineering Section (RES), Safety Engineering Diuision (SED), Oarai Engineering Center (OEC),

Power Reactor and Nuclear Fuel Deuelopment Corp. (PNC), 4002 Narita, Oarai-machi, Ibaraki, 311-1393, Japan

Received 13 March 1998; received in revised form 5 May 1998; accepted 29 December 1998

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, ΔThc=5°C, 10°C and r =Vcold,exit/Vhot,exit=1.0 (isovelocity), 0.7, 0.5 (non-isovelocity) respectively. The typical Reynolds number was ReD =1.8 ×104, where D is the hydraulic diame- ter 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 (Vexit =0.5 m/s) with ΔThc=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 v|2.25|, centered about the cold jet. An estimate of the turbulent heat flux distribu- tion 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. © 1999 Elsevier Science S.A. All rights reserved.

* Corresponding author. Tel.: +81-29-267-4141; fax: +81-29-266-3718.

E-mail address: kimura@oec.jnc.go.jp (N. Kimura)

0029-5493/99/$ - see front matter © 1999 Elsevier Science S.A. All rights reserved. PII: S0029-5493(99)00006-0

1. Introduction

   Thermal striping refers to random thermal cy- cling of reactor structures and components as a result of fluid– structure interaction; that is, strip- ing 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 flow- ing through and exiting the reactor core. The net result of striping is undesirable since thermal fa- tigue of materials can lead to structural and mate- rial failure. Thermal striping as a phenomenological problem in LMFBRs was al- ready 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 interac- tion, 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-hy- draulic 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 stud- ied, 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 experi- ments. 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 inter- est 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 exitvelocity relative to the central jet. The three-jet arrangement is a simplified simulation of hot and cold flow channels in a LMFBR core. An under- standing of thermal striping or rather the convec- tive mixing is one of the key issues in the safe design of the LMFBR. Experimentally, one objec- tive of the study was to demonstrate the appli- cability of the ultrasound velocity profile (UVP) monitor for velocity measurements. By applicabil- ity we mean velocity measurements in the flow field of relevance. Subsequently, we first obtained and evaluated the hydrodynamic information con- cerning the nature of mixing between thermally- stratified jets. Then with the addition of temperature data we were able to assess the ther- mal-hydraulics of mixing process.

2. Experiment

   1. Experimental facilityFig. 1 shows the experimental loop including the test section. Except for the test section, theFig. 1. Schematic of experimental loop.Fig. 2. Schematic of test section.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/ex- changer 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 into the loop and finally a general purpose laboratory water supply tank. Several turbine flowmeters as well as orifice plate type devices, a system of valves and all the connecting piping are as depicted.A more detailed view of the test section itself is shown in Fig. 2. The test section is immersed within a rectangular tank measuring 2438W× 2438H×671D (W is width, H is height, D is depth, all mm), and the test section itself is a partially enclosed rectangular region measuring 400W×950H×176.5D. As noted in the topview, two acrylic plates sandwich the four rectan- gular blocks thereby restricting the spread of the exiting jets in these directions. The rectangular blocks and plates defined three exits, each measur- ing 50 ×176.5 mm. The equivalent hydraulic di- ameter was D =35.7672 mm. The idea was to constrain the jet to a finite width and to ‘view’ it as quasi two-dimensional (planar) within this ge- ometry. The right and left sides are open so that even with an overflow mechanism at the top of the test section there may be some recirculating flow through the sides. A prominent feature of the tank is the large viewing glass windows on both the front, back and right side of the tank. This feature was included primarily for laser-based measurements and flow visualization techniques. Below the test section are three rectangular chan- nels defined by four equally rectangular blocks. The central channel functions as the ‘cold’ jet supply while the adjacent two are ‘hot’. The hot and cold jets are supplied from separate sources, the cold source being centrally situated, flowing first through an expansion, a grating and then through a flow constriction. The hot source is on the other hand supplied from the right-hand-side into a lower chamber. The flow then weaves its way past the cold pipe and enters symmetrically through a one-sided rectangular constriction. The exit of the nozzle is a block elevated 45 mm from the reference groundplane of the tank.The other prominent components of the testfacility is the traversing thermocouple array and the ultrasound transducer holder affixed to the left arm of the traversing mechanism. A schematic is shown in Fig. 3 along with the exit blocks. The moving mechanism consists of two vertical and parallel pillars (OD 45 mm; only left is shown), between which a ‘bridge’ served as a mounting bracket for thermocouples. This bridge is fixed and moves up and down with the pillars. The pillars are traversed externally from above the tank by an electric motor. The traversing thermo- couple array consists of 39 thermocouples (T/Cs) facing vertically downward and horizontally spaced 5 mm apart over a 190 mm span. The last 5 mm of each of the 39 thermocouples are directly exposed to the flow, while beyond this point the T/C is insulated for a length of 50 mm. The T/Csare threaded and bonded to the horizontal bridge and the lead wires are contained either in the right or left pillars. The two arms exit out the top of the rectangular tank. The thermocouple are T- type, constantan copper– nickel with an expected measurement error of 0.5°C. Operationally three T/Cs malfunctioned (Nos. 5, 6, 14, numbering from left) and could not be used for data acquisition.Velocity measurements were taken using the Met-Flow Model X-1 ultrasound velocity profile (UVP) monitor (Met-Flow SA, Lausanne, Switzerland) with a single, Delrin-encased (tem- perature limit v80°C) piezo-electric transducer operating at 4 MHz. The transducer had an ultra- sound beam diameter of 6 mm with a beam spreading angle of approximately 3° over 75 cm. The UVP is an ultrasound Doppler velocimeter, working on the principle of echography; that is, the position and velocity information are evalu- ated respectively from the detected time-of-flight and the Doppler-shift frequency at the detected position, within each of 128 ‘coin-like’ volumetric elements along the beam’s path, during 1024 time intervals. Thus at each time interval, a componen- tal velocity profile, based on 128 points, is con- structed along the measurement line (ML) of the ultrasonic beam. By componental it is understoodFig. 3. Schematic of instrumentation set-up. Close-up of the UVP transducer orientation and traversing thermocouple ar- ray.to mean that the velocity vector oriented either toward or away from the face of the transducer, determined(from the sign of the Doppler shift. The real-time corresponding to 1024 measurement intervals is adjustable depending largely upon the preference (and experience) of the user, though it should be based on the phenomenon of interest in the flow; that is, based on estimates of the time- scales associated with various transport phenom- ena, the user is able to select either a short or long time span between measurements. The UVP can thus detect time-dependent phenomena during a minimum time-span of 30 ms to minutes and hours. The device has been developed and tested in thermohydraulic applications, most notably by Takeda (1986, 1991, 1993).The ultrasound is reflected from tracer parti- cles, typically a plastic powder with a nominal size of 50 – 100 µm (ρ=1.02 kg/m3), that are added to the test medium (water). One should note that the inherent assumptions in using this measurement technique are that: (1) the tracer particles accu- rately reflect the velocity profile of the liquid state and (2) the modification of the flow field due to addition of tracer particles; that is, the particle– fluid interaction, is of minor consequence to the measured profile. Additionally, it is assumed that particle-to-particle interactions are negligible. We realized this by using a low concentration of tracer particles, on the order of 100 g per 4000 l (3988) of water. Finally, regarding the former, we assume that there is no slip (relative) velocity between tracer particle and liquid; that is, the particle moves exactly as a fluid element would, as dictated by the initial and boundary conditions of the flow. As for the positioning of the transducer, it was held in place by a short piece of pipe through which the transducer was inserted (and held) while the output signal traveled through a 4 m long cable. The typical measurement time for128 spatial×1024 temporal points, was on theorder of 1 – 3 min.
   2. Conditions of UVP and temperature measurementsFor the data presented in this paper, the aver- age exit velocity of both the single- and triple-jetTable 1Experimental conditions

      Case	T05 V0505	T05 V1010	T10 V0505	T10 V1010	T05 V1005	T10 V1005	T10 V1007	T05 V1007
      Hot jets
      Velocity (m/s)	0.5	1.0	0.5	1.0	1.0	1.0	1.0	1.0
      Temperature	30	35	42	42	30	40	40	32
      (°C)
      Cold jets Velocity (m/s)	0.5	1.0	0.5	1.0	0.5	0.5	0.7	0.7
      Temperature	25	30	32	33	25	30	30	27
      (°C)
      Discharged	5	5	10	9	5	10	10	5
      temperature
      difference
      (°C)
      Discharged ve-	1.0	1.0	1.0	1.0	0.5	0.5	0.7	0.7
      locity ratio
      Wcold/Whot

      configurations were 0.5, 0.7, 1.0 or 2.0 m/s with an estimated error of 0.1 m/s. The temperature difference between the cold and each of the hot jets was either 5°C or 10°C in all cases with an estimated, conservative error of 0.75°C. UVP measurements were conducted with the transducer fixed at either the right (R) or left (L) locations with respect to the jet(s) (see Figs. 2 and 3). Measurements were taken axially, along the z- axis, at 5-mm intervals up to approximately 550 mm above the imaginary ‘0’-plane in most cases. For all the data present here, the UVP transducer was oriented at an angle of 10° with respect to the horizontal. The selection of the 10° angle was an experimental compromise between having a suffi- cient number of axial locations, which we sought in order to follow the flow development, and the inclusion of the larger, axial vector component relative to the horizontal component of the actual jetting flow. Table 1 summarizes the experimental conditions covered in this paper. UVP measure- ments were restricted to case T05V0505.

3. Results and discussions

   1. Photographs and uideo imagesWe first present in Fig. 4(a) and (b) digitized image sequences of respectively, the single- andtriple-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 temperature difference conditions. Note that qualitatively some flow structures are evident and that some contrasts such as in charac- teristic lengths appear in (a), (b) and (c). Since a normal speed video camera was used to record these images, some fast flow phenomena could not be captured. Nevertheless, it is clear from the figure that our triple-jet has a spatial (x,z) and temporal (time) dependence. Note that in the present set-up the axial coordinate is the z-axis (streamwise) and the spanwise (transverse) dis- tance is the x-axis. Finally in order to facilitate our presentation, we refer to the buoyant jets as the ‘hot’ jets and the non-buoyant, central jet as the ‘cold’ jet.
   2. UVP uelocity profiles: single-jet and triple-jetFig. 6(a) shows a representative set of average velocity profiles of the single-jet at z-locations taken by the UVP. The profile shown is that of the velocity component at 10° to the horizontal;that is, nearly the spanwise component. The profi- les have been chosen to clearly display the changes with downstream locations. The abscissa depicts the 128 channels (0 – 127) along the ultra- sound beam, a distance equivalent to 284 mm, with the centerline taken as the origin (x/D =0). In Fig. 6(b) we show one profile (at z =45 mm) and its associated standard deviation profile in order to explain details of the profile itself. The actual profile as measured by the transducer de- picted in Fig. 3 is the inverted image of Fig. 6(b);that is, recall that with respect to the transducer, flows coming toward it are ‘ −’ (negative) and those flowing away are ‘ +’ in terms of the sign of the Doppler shift. The inverted profile does not, however, change in any way the information con- tent of the depicted velocity profile. We thus see that a prominent feature is the peaked, jet-like profile in the central region. Additionally, to ei- ther side of the center is the entrained-flow re- gions which show flow of approximately equal magnitude and on-average of opposite sign withFig. 4. (a) A sequence of three snapshot images digitized from video of the single-jet. (b) A sequence of three snapshot images digitized from video of the triple-jet.