Department of Aeronautics & Astronautics

College of Engineering

UNIVERSITY OF WASHINGTON

Technical Note No. 69-1

February 1969

A SMOKE GENERATOR FOR LOW

SPEED WIND TUNNELS

by

Shojiro Shindo

and

Otto Brask

1ACKNOWLEDGMENT

The authors wish to acknowledge the generous help of Mr. Robert S. Nielson in every

aspect of this project. Much gratitude is also due the staff of the University of Washington

Aeronautical Laboratory (UWAL) for assisting and allowing us to use the facility.

In addition the authors would like to thank Mr. Robert Blair for his kind assistance in the

design of the Power supply. The Army Research Office Durham Grant DA-ARO-D-31-124-

G809, the University of Washington Department of Aeronautics and Astronautics, and the

Aeronautical Laboratory for their fiscal support.

SUMMARY

It has been the dream of the aerodynamicist to be able to see the the actual flow

characteristics around a three dimensional model in a wind tunnel. The smoke generator

described in this report can be used in any low speed wind tunnel in conjunction with other tests.

The device produces a non-toxic, non-corrosive, dense white smoke suitable for observation and

photography.

2INTRODUCTION

In this field of aeronautical science, the flow visualization has been a nebulous observation

simply because air is invisible. The use of saw dust, tufts, fluorescent dye in oil with black light, a

recently developed laser beam and smoke, are some of th methods and materials used to attempt

flow visualization.

The use of saw dust increases the difficulty of maintenance in the wind tunnel and its

associated equipment. The tuft and oil methods show flow on the surface of the model. the laser

beam has not yet been perfected as a usable tool for this purpose. Whatever the method used, the

substance introduced into the tunnel should be non-corrosive, non-toxic, and the equipment must

be safe to handle.

The use of smoke appears to combine all the best characteristics of available methods. In

the early days, rotten wood was the popular source of smoke, (Ref. 1 and 2). However, smoke

produced by the burning wood was an eye irritant and caused an accumulation of tars.

The technique of evaporating oil by various methods was introduced to eliminate some of

the problems of the smoke, (Ref. 2). The study of smoke generators has also been reported in

England and Australia, (Ref. 3, 4, and 5). In the United States, Princeton University produced

smoke using kerosene, (Ref. 6).

All of these methods require rather complex set up of equipment for the reason of either

safety or health. Sellberg of Sweden wrote a report on the subject of his smoke generator, (Ref.

7). He used what he called white spirit to produce the smoke. The design of his smoke

generator was modified and several commercially available liquids were tested at the Department

of Aeronautics and Astronautics, University of Washington.

This report presents the design and results of the smoke generator now in use at the

University of Washington.

3THE SMOKE GENERATOR

The principle of the device is to force oil through a small diameter tubing which is heated

by electric current. The oil vaporizes in the tubing and quickly condenses to form a visible

smoke, as it comes out of the open end. The probe can be fixed or hand held at any desired

location.

1. Probe

The probe itself is a 0.060 inch O.D., 0.010 inch wall stainless steel full hard,

hypodermic needle about 75 inches long placed inside of a 0.38 inch O.D. steel tubing. A

number of ceramic beads are slipped onto the stainless steel tubing to electrically insulate

it from the outer tubing. The 0.38 inch O.D. outer tubing is made to contact the needle at

about 8 inches from the tip. The needle is heated by the electric current of about 10 to15

amps for about 60 inches of its length. The current is applied near the base of the

assembly to as to utilize the 0.38 inch outer steel tubing as a conductor. Figure 1 shows

the schematic diagram of the smoke generator. The probe design allows for the needle to

expand linearly due to the heat. The amount of expansion is made visible at the base by a

marking on the tubing. The marking, the end portion of the needle, the power lines are all

housed in a handle made out of a non-heat conducting material, thus, making it convenient

to hand hold the unit if so desired.

The device can produce a good smoke with a probe configuration other than

straight, such as a coiled form of approximately 0.5 inch diameter and 4 inch long, or any

other insulated shape to fit the test configuration. This type of arrangement is convenient

if the long stem of the smoke generator cannot be introduced into the test section from

outside.

The probe tip can be made to produce a single, or up to 4 stream of smoke about 2

inches apart. The system described here cannot control the smoke at any one individual

tip when a multiple tip configuration is used.

2. Power Supply

4Figure 2 shows a dual power supply package of which each unit consists of a

Variac anda 1.5 KVA, 115:230 volt transformer connected in a step-down fashion. This

system, with 115 V. AC, makes it possible to control the element temperatue by changing

the voltage across the hypodermic needle from zero to about 50 volts. Since the

resistance of the tubing used here is about 3 ohms, the current is not more than about 15

amps. The power package shown in Figure 2 weighs approximately 100 lbs. which can be

reduced to about 50 lbs. if only on unit system is needed.

3. Fuel

Several different fluids were tried to produce the smoke. Kerosene, cleaning

solvent, Carnea oil, and paint thinner were forced through the heated hypodermic needle

by compressed air. Some of the fluids made a deposit of tar, and others produced a smke

that was not dense enough. The best result meeting the requirement of beinga non-toxic,

non-corrosive, dense smoke, was obtained when a produced called Type 1964 Fog Juice

was used. This product has a boiling temperature of approximately 530 F, containspetroleum hydrocarbon and is commercially available from any theatrical supply dealer.

4. Reservoir

The device includes a fuel reservoir which is equipped with a pressure regulator

and a fuel flow control valve. Compressed air is supplied to the reservoir through the

pressure regulator, and when the valve is opened, the fuel flows through a flexible plastic

tubing connected to the hypodermic needle.

5. The Model and Background

In order to obtain a good photographic record, the model and background should

be painted flat black to achieve the maxmimum contrast with white smoke produced by

this device.

6. Operation of the Smoke Generator

5Specific instructions on the amount of voltage, fuel flow rate, and pressure

qrequired to produce a saisfactory smoke are difficult to include in this paper because

theyare highly dependent on the individual smoke generator and the environment of

experiment.

This section describes the general procedure of how to operate the device.

Increase the voltage to about 70% of the transformer output and wait for a fewseconds to heat the probe.

Apply compressed air at about 30 psi.

Supply fuel by cracking the valve.

The amount of fuel required is very small, being in the neighborhood of 1 ounce per

minute. If hot oil comes out of the prob tips instead of smoke, generally it is the sign of

low temperature. Increase the voltage to improve the condition. If the smoke pulsates,

generally it is the sign of low air pressure. Increase the pressure to obtain a good smoke.

This condition can also be remedied by decreasing the voltage. It is noted that only a

good combination of the three controls can produce a satisfactory smoke.

6RESULTS AND CONCLUDING REMARKS

The smoke produced by passing the Fog Juice through a heated long hypodermic needle

is non-corrosive, non-toxic, and has a pleasant odor. The smoke is dense, white, and cool, being

about 150 F at about 1.5 inches away from the probe tip. The smoke streams appear to holdtheir definition at an air speed of about 100 mph, Figure 3. The smoke is also useful at very low

air speeds. Figure 4 shows the smoke streams at an air speed of about 14 mph.

The method described here has a positive control of smoke on or off. With the probe

configuration of 4 tips, a half gallon of the Fog Juice can produce a continuous smoke for about

an hour. By proper adjustment of the element voltages, the amount of fuel flow and pressure, this

generator can produce a continuous white dense smoke suitable for observation and/or

photographs as long as the fuel lasts.

ADDENDUM

Type 1964 Fog Juice is a Mole-Richardson Product. As of January, 1995 the prices were:

$15.35/quart, $26.61/gallon from a local supplier. The price of Rosco fog juice was $14.95/liter,

$53.00/gallon. Please contact the manufacturers directly for the latest prices and availability:

Mole-Richardson Company Rosco

937 N. Sycamore Avenue 1135 N. Highland Avenue

Hollywood CA 90038 Hollywood CA 90038

213-851-0111 213-462-2233, 1-800-767-2652

7

8Figure 2. Dual Unit Power Package

9Figure 3. Smoke Streams at Approximately 100 mph

Figure 4. Smoke Streams at Approximately 14 mph

10

REFERENCES

(1) Ludington, C.T., Smoke Streams, Coward-McCann Inc. New York, 1943.

(2) Lippisch, A. M., Flow Visualization, Aeronautical Engineering Review, February 1958.

(3) Salter, C., Multiple-Jet White-Smoke Generators, Aeronautical Research Council R &

M No. 2657, 1953.

(4) Maltby, R.L., Flow Visualization in Wind Tunnels Using Indicators, AGARDograph

70, 1962.

(5) Hodges, A.E. and Pound, T.N., Futher Development of a Smoke Producer Using

Vaporized Oil, Aerodynamics Note 233, Australian Defense Scientific Service

Aeronautical Research Laboratories, November 1964.

(6) Hazen, D.C., Some Results of the Princeton University Smoke Flow Visualization

Program, Fifth International Aeronautical Conference, Los Angeles, 1955.

(7) Sellberg, L.D., Smoke Generator Type Lorinder for Flow Visualization in Low Speed

Wind Tunnels, Technical Note KTH AERO TN 55, Departmment of Aeronautical

Engineering, Royal Institue of Technology, Stockholm 70, Sweden, 1966.