Direct contact heat transfer study between hot gas and water spray
(Fire in Tunnel)

Activity description
Experimental facility (Test Section)
Applications
Photo gallery
What we have done previously...

Activity responsible: Dr. Francesco D'Annibale
Address: ENEA C.R. Casaccia, Institute of Thermal Fluid Dynamics, Via Anguillarese 301 (S.P. 092), 00060 S. M. di Galeria RM, Italy 
Phone: +39 06 3048 6467 Fax: +39 06 3048 3026 

Email: dannibale_f@casaccia.enea.it



Activity description

The aim of the experiment is the evaluation of direct contact heat transfer between high temperature air and water droplets. In order to get 'clean' experimental data, also more easily predictable with theoretical models, we use a system which allows the generation of a water spray characterized by a uniform value of the droplet diameter distribution (monodimensional drop diameter distribution of the spray).

The working principle of such a spray is based on the varicose break-up of the liquid jet coming out from the nozzle (Reyleigh-Weber instability). Imposing a periodic disturbance to the liquid jet, with given amplitude and frequency, it is possible to make the jet broken at a fixed location, also getting droplets with the same diameter (depending on the liquid velocity nozzle diameter, and disturbance frequency). Scheme of the system.

Using the spray developed for the present experiment, we obtain a droplet diameter ranging from 0.29 to 2.1 mm, with an initial velocity ranging from 1 to 8 m/s. (click to see the droplet characteristics)

The technical solution adopted for the droplet generation (wave disturbance induced by means of a small piston connected to an external mechanical vibrator), allows to inject the droplets into the hot gas environment, at a temperature of more than 900 deg.C. 
Picture of the system (32K).

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Experimental facility
 
 

ImpFIT.gif (16 K)





BOIL facility consists of a hot air and a water line. 
Air is supplied either from pressurized vessels or from the air network, and the gas flow rate is operated through the regulation of the pressure upstream a calibrated orifice where the sonic flow conditions are reached. Air is then heated to the test temperature and injected into the test section. 

sdp.gif (15060 byte)

This latter consists of a rectangular channel (h = 60mm, l = 20 mm) obtained using refractary low-density bricks (480 kg/m3) heated to the same temperature as the inlet gas. The air temperature is measured using 3 thermocouples before the contact with droplets and 10 thermocouples after the contact with the droplets

Picture of the channel (20 K): outlet thermocouples are in front, while inlet ones are on the back
Pictures of the test section: 
Bare bricks (28 K)
Bricks with the heater, glass windows and the support for the air heater (60 K)
Bricks with the insulation and instrumentation (68 K)

Demineralized water, coming from a pressurized vessel, runs through a Coriolis-meter, and is injected into the test vessel through the spray above described. 

Droplets, after the interaction with the hot gas, are gathered by a ceramic catcher, where two thermocouples for the collected water temperature measurement are placed. 
Droplets are monitored using two windows (made of high-temperature glass, Tmax=1200 deg C), in order to verify the uniform size and shape.

We use five sprays with  different holes.
Droplets generated from the spray have the following characteristics

Dopt
fopt
 
[mm] 
[mm] 
[kHz] 
24 
0.15 
0.28 
3.5 - 13
16 
0.25 
0.49 
1.6 - 6.4 
0.4 
0.78 
0.55 - 2.5
3
0.6 
1.0 
0.55 - 3.3 
1.0 
1.3 
0.5 - 3.0 
where:
 
n is the number of holes 
d is the diameter of the hole 
Dopt is the droplet diameter under the most favourable condition
 
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Applications

The research if finalized to study the cooling of hot gas generated by fire in tunnel accidents. In these cases in fact it is important to estinguish the fire and to reduce the hot gas expansion along the tunnel using emergency sprays in an optimal way. In order to optimize these systems, it is necessary to know the heat transfer coefficient for the direct contact between droplets and hot gas. The experiment is finalized to obtain this data as a function of droplets dimension and velocity, and gas velocity and temperature.

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Photogallery

Pictures of the test section during the construction: