A. Winyard

D. Beastall

English

MAGiC ARC Repository

C.P. No. 106 (14.400) (14,562 (Addendum)) / A.R.C. Technical Report MINISTRY OF SUPPLY AERONAUTICAL RESEARCH COUNCIL CURRENT PAPERS A Frost. Point Hygrometer for Supersonic Wind Tunnels BY D. Beastall and A. Winyard LONDON: HER MAJESTY’S STATIONERY OFFICE 1953 Price Is. 6d. net C.P. NO. 106 Technwal Note No. Aero 2114 July, 1951 ROYAL AIRCRAFT ESTABLISGEE - A frost poznt hygrometer for supersornc wind tunnels D. Beastall and A. Wlnyard Thu note describes a frost point hygrometer sultable for measurrng the water vapow content of the au in supersoruc wind tunnels at any stagnatux press- vlthln their present range of operation. It uses co2 as a coolant and 1s economxal m construction and operation. LIST C!Y CONTENTS 1 Introduction 2 2 Descrlptlon of hygrometer 2 I)etennatlon of the water vapour content of a sample of au : Further development References LIST OF ILLUSTRATIONS Frost pomt hygrometer I Molller diagram for air saturated with water vapour at vapour- solzd equrllbrlum for sub-freezmg temperatures Vapour pressure of co2 : Hygrometer uwtallatu3n 4 -l- 1 Introduction - In order to avoid spurious readings in supersonic wind tunnel tests lt 1s essential to control the humidity of the air in the tunnel to some small value. Thm note describes a frost point hygrometer suitable for measuring humidities down to at least 0.0005 lb of water/lb of air over the range of pressures at whxh variable density wind tunnels operate. This requires a coolant capable of giving a minimum temperatti of -40°C if the tunnel is to be operated at pressures down to 2.5 lb/sq in ,aa;. with control to any intermediate temperature up to O°C or above. coolant used is carbon dioxide throttled from a high pressure cylinder. The main advantages of this hygrometer are:- 1. Simplicity and cheapness of construction and operation. 2. Speed with which a frost point oanbe obtained (especially desirable for intermittent tunnels). 3. Freedom from leaks (essential at pressures less than atmospheric). 4. Availability of coolant. 5. Fine degree of control of temperature. The hygrometer described here is in operasion in the 9" x 9" sver- sonic wand tunnel at the R.A.E. and has been used for a short time without any serious troubles. A-more elaborate form employing CO2 coolant is in use in AIReriC. and 1s described in Ref. 1. 2 Description of hygrometer Flg.1 1s a detailed drawing of the hygrometer. It consists essen- txd3.y of an arrangement for cooling a metal disk over which 1s passed the air whose humidity is required. The formation of frost is noted v~ually; ' the temperature of the disc at which frost is deposited on it is measured by a thermocouple, and the pressure of the air in the hygrometer is measured directly from a gauge. From this information the water content of the 8x- can be derived using the chart in Fig. 2 reproduced from Ref. 2. iviore specifically the construction and operation of the hygrometer 1s as follows:- It is connected (see Fig.1) to a high pressure carbon dioxide cylinder, the CO2 expands through a jet formed by a 6" length of 0.5 mm outside dia- meter stainless steel hypodermic tubing and impinges on the undersIde of a brass disc. Control of the temperature of the gas is by means of the pres- sure lri the expansion chamber and this 1s regulated by the control valve (a needle valve). Fig. 3 shows the-relationship between the pressurc.and temperature of the CO2 when expanded from high pressure. It is essential during operation to keep the 'pressure in the expansion chamber above the tro.Ple point- (approximately 5 atmospheres) in order to avoid choking the Jet with solid CO2. This, therefore, is the lower lzmit of operation of the hygrometer and is -70°F (-56.7OC). The expansion chamber pressure is recorded on an air pressure gauge. The tunnel air is directed onto the top of the disc from a &" diameter inlet pipe and is returned through the air outlet. To facalitate the observation of frost ths top of the diet has a matt black finish. The ~I.SC -2- IS made of brass and is insulated from the body of the hygrometer by fibre washers to prevent excessive dissipation of the coolm~, effect of the C02. The temperature of the disc is measured by means of a copper-constantan thermocouple JLUICtlCn (not shovm in Fig.1 ) soldered to the upper surface of the alsc. The hole through which the thermocouole leads were taken was made airtLght by the use of araldite. The loser cylindrical part Of the hygrometer was originally intended as a filter to reduce the,w.ater content of the CO2 passing through the jet and thus reduce the possibility of icing up. It was found to be superfluous but has been left simply as a housing for the 6" length of 0.5 mm tubing comprising the jet. Fig.4 gives dzagrsmmatically the layout of the installatlcn on the tunnel. Pipes are connected to the settling chamber and return circuit of the tunnel and thus provide a pressure difference for passing the au‘ through the hygrometer. The pressure gauge (in this case a mercury bare- meter capable of reading pressures in the range of tunnel operating stag- nation pressures) gives the air pressure in the hygrometer. The thermc- couple 1s connected to a -voltmeter (scale O-4 mv) the reference J~C- tzon berng mmersed U-I transformer~~l5.n a thermos flask. The thermc- couple was caLrbrated so that the &sc temperature ccrrespcrx%ng to the rmllurdLtmeter rearlug was knovm. Thrcughcut the-mstallat~on attextron has been paid to the ewtion of leaks especially on the inlet side of the alrlrne to the hygrometer and the hygrcweter itself has been subjected to a pressure test at 1,500 lb/ sq in. to ensure that tnere is nc leak of CO2 to the upper surface of the alsc. 3 Ikte-fitlon of the water vapour content of a samole of air Allow the tunnel air to flow through the hygrometer by opening the taps m the inlet ana outlet pipe lines. Yith the control valve closed, open the CO2 cylinder valve, the expansion chamber pressure will thenbe the cylin- der pressure and no cooling of the disc will take place. Now by opening the control valve slowly the temperature of the disc can be lowered until frost appears; the reading sf the millivoltmeter at which this occurs is then notea. The temperature of the reference JwCtlOn and the air pressure in the hygrometer are then required. When the frost point temperature has been calculated, the water vapour content can be derived using Fig.2. The operating tune for obtaining a frost point should not be more than half a mmute . When closing the CO2 cylinder valve ensure that the control valve is set above 5 atmospheres to prevent blocking of the Jet by sohd CO2. 4 Further development It should be realised that the arrangement described in this note is the outccnle of a very short development programme and refinements may be made as experience is gained in its operation. Only the naln dimensions are given in Fig. 1 since the successful operation seems to depend only on a few dimensions. One ur.portant point to note in the construction is that the CO2 outlet should be large compared with the size of the jet to prevent solid CO2 blocking the cutlet. The only maintenance at present carried out is the periodical drying out of the hygrometer to prevent the accumulation of water deposited by the C02. The upper and lower limits of cylirxder pres- sure which cover the range of satisfactory operation of tnc hygrometer are not known at present. Perhaps the only major modification which seems desirable is the installation of an automatic method of recordmg frost to eliminate the human error in visual observation. Acknowledgement The construction and installation of the hygrometer are due to Mr. N. Searle who also offered helpful suggestlow during Its development. -3- Ref. No. Author 'Iltle, etc. 1 T.L. Wheeler 1nstrumerit for measLr1ng Kiter-vapour H. cl. lkhhhan conter,t l.n anator's oxygen. osm 5151. Iiay 1945. 2 R.V. Hensley Xolller dxqrams for air saturated with water vapcw at low temperatures. NACA T.N. 4715. Addendum to Tech. Note No. Aero 2111~ November, 1951 ROYAT.. AIRCR&?T'ESTABLISHMXNT - - A frost pou-x hygrometer for supersonx wux? tunnels D. Beastall and A Yilnyard ~.DDENDUM --- Smce the ISSUE of Tech. Note Aerc 2114, satisfactory operation of the hygrometer has been found to depend on the temperature of the CO2 cylmder beuq above a certain value which 1s roughly 25cC. Thu temperature determines whether any moisture m the CO2 vrlllbe deposited as ice ln the Jet of the hygrometer during operation, thus causing blockage. If we treat the flow of tke CC2 as a sunple fluld flow problem the mummum temperature attainable in the Jet can be expressed m terms of the temperature (To) of the CO2 in the cylinder. The Jet 1s acting as the throat of a superscmc no~z le and tr‘e ten>erature m the Jet wx.11 bc that corresponding to scn~c velobxty tf tnc W2. Tnx nxuTl3lUII1 temperature, T, 1s gIvenby the relation T = To (I + $1 -' = To x (1.165) -' (1) Thus If Tc I.S ‘above 45'C, T wlllbe above O'C and xx ~111 not form m the Jet. Thx. however, assumes no supersatwatlon of the water vapour In the C02. In practxe it has been found that 5uccessful operation of the hygrometer 1s pcsslble crlth cylinder temperatures of round about 25% 0~‘ above. -4- kt.2078.CP106.K3. Fwntad rn Grant BTztatn. TO CC(NTROL VALVE EQUALLY SPA ON 3.25” P.C 0 FIG. I. AMERICAN PROJECTION. MAT!= BRASS SCALE:- FULL SIZE. TO AIR PRESSU’;)E GAUGE. AIR OUTLET. $” GLASS PLATE. I TO CdNTROL VALVE. I , - -  EXPANSION CHAMBER. CROSSLITE ;IOINTING. HOLES TAPPED FOR I” LENGTH. ALUMINIUM WASHER. .0*5 1.1.M. S.S TUBING. TO CO2 CYLINDER. FIG. I. FROST POINT HYGROMETER. -., ‘,‘.. _‘- - ._,1 &- _ _ I I I’ I FIG.2. MOLLIER DIAGRAM FOR AIR SATURATED WITH WATER VAPOR AT VAPOR-SOLID EQUILIBRIUM FOR SUBFREEZING TEMPERATURES FIG.3 PRfSSURE - ATMOSPHERES. FIG. 3. VAPOUR PRESSURE OF C02. FIG.4. THERMOMETER CON STANl-AN T THERMOCOUPLE REFERENCE JUNCTION AIR FROM SETTLING CHAMBER TAP 1 MILLIVOLTMETER. COPPER ‘HERMOCOUPLE ‘OINT CONTACT 3N DISC) HYGROMETER IN EXPANSION CHAMBER GAUGE SHOWING AIR PRESSURE IN HYGROMETER. SUPPLY CYLINDER FIG.4. HYGROMETER INSTALLATION. C.P. No. 106 (14.100) (16,562 (Addendum)) A.R.C. Technical Report Crown Copyright Reserved York How. ffiwsway, UXKION. w c.2. 423 Oxford Street, LONDOW w.1 P.0 Box 569, L‘ZJNDON, se.1 13a castle street, amNa”aoH, 2 I St Andrew’s Crescent, CARD~P 39 Kmg street, MANcma. 2 Tower Lam. nmsnn., 1 2 Edmund Street, BIIUWOH*M. 3 80 t2hKhestcr street. BELFASI or from any Bookseller 1953 Price 1s. 6d. net PQJNTED IN GREAT BR,TNN S.O.Code No. 23-9007-6 

A frost point hygrometer for supersonic wind tunnels

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A frost point hygrometer for supersonic wind tunnels

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