READ THESE TERMS AND CONDITIONS CAREFULLY BEFORE USING THIS WEBSITE. https://nrc-publications.canada.ca/eng/copyright
Vous avez des questions? Nous pouvons vous aider. Pour communiquer directement avec un auteur, consultez la première page de la revue dans laquelle son article a été publié afin de trouver ses coordonnées. Si vous n’arrivez pas à les repérer, communiquez avec nous à PublicationsArchive-ArchivesPublications@nrc-cnrc.gc.ca.
Questions? Contact the NRC Publications Archive team at
PublicationsArchive-ArchivesPublications@nrc-cnrc.gc.ca. If you wish to email the authors directly, please see the first page of the publication for their contact information.
NRC Publications Archive
Archives des publications du CNRC
This publication could be one of several versions: author’s original, accepted manuscript or the publisher’s version. / La version de cette publication peut être l’une des suivantes : la version prépublication de l’auteur, la version acceptée du manuscrit ou la version de l’éditeur.
Access and use of this website and the material on it are subject to the Terms and Conditions set forth at
Combined heat and moisture flow-philosophy and review of a Canadian
Research Program/Ecoulements de chaleur et d' humidite combines
Hutcheon, N. B.
https://publications-cnrc.canada.ca/fra/droits
L’accès à ce site Web et l’utilisation de son contenu sont assujettis aux conditions présentées dans le site LISEZ CES CONDITIONS ATTENTIVEMENT AVANT D’UTILISER CE SITE WEB.
NRC Publications Record / Notice d'Archives des publications de CNRC:
https://nrc-publications.canada.ca/eng/view/object/?id=caa178f0-8271-4d63-9f33-a55502973d6f
https://publications-cnrc.canada.ca/fra/voir/objet/?id=caa178f0-8271-4d63-9f33-a55502973d6f
Ser
TH1
N21r2
no.
119
c. 2
COMBINED HEAT AND MOISTURE FLOW
PHILOSOPHY AND REVIEW OF A
CANADIAN RESEARCH PROGRAM
N. B. HUTCHEON
I~ e s e a r c h
PaperNo.
119of the
Division
of
Building Research
National Research Council, Canada
Reprinted
from
Extrait des
PROCEEDINGS OF THE 10th INTERNATIONAL CONGRESS OF REFRIGERATION
COMPTES RENDUS D U 10' CONGRES INTERNATIONAL D U FROID
Copenhagen 1959
Copenhague 1959
Volume
1
Tome
1
COPYRIGHT PERGAMON PRESS LTD., LONDON, 1960
OTTAWA
Pricc 10 Cents
FEBRUARY
196 1Combined Heat and Moisture Flow-Philosophy and Review of a Canadian
Research Program
N . B. HUTCHEON, Assistant Di~ector,
Division of Building Research, National Research Co~lncil, Ottawa, Canada
S O M M A I R E . Cc rnppar.t pa.r.re erl rcvlte le p r o g r a n ~ ~ n c rlc recherche.r d ~ c Cnnndn d c p ~ i i s 12 ntu et 1e.r the'arier q n i .re .rorrt diveloppker pnrnlldlenrent. Lc.r calclrls rle l'dco~rlenre~rt d e chnlelrr 2 lraver.r 1e.r nznte'rinlrx d e con.rtvlrction .ront base's .rur l'hypolhdre qlre 1e.r rrrnte'ria~/x 1rti1i.rk.r .rorrt .rec.r. Cepenrlant il y cr prerqrre tolijolrrs d e I'eau en pratiqrre. L n prk.rence d e I'enrr r1rrn.r les mcrte'rialrx porerrx crPe dczns l'e'coule?nent d e chnleur rrne conzplicntio~r qlre savants et ingknierrr.r rz'onl pas enc0r.e pu arriver 2 trailer rle f d ~ o t z 1 . ~ ~ l i 0 t I e l l ~ . 1x.r expkrience.r 2 ce jorrr irzdiqlrent qlre pour de.r grndie~2t.r d e tenzpir.rrtrrre et d e pres.rion d e v a p e u ~ cotnbini.r, la transfert d'br~tniditk .re fait par Irne di.rpo.ritior2 se'rie-pnrnlldle d e pores rernp1ie.r d e vnpero. et remp1ie.r de liqrrirlc, avec e'vaporation et condenratiolz cotztinlre entre 1e.r 1irnitc.r d e liqlride et d e vnperrr. U n e Ctlrde nnnlyliqrre rigolrrelr.re exigerait des connaissnnces gkomktriques prkcises et compldies sur lu gkonze'trie dlr systdme d e pores. CJc.rt e'videmnzenl itnpos.rible erz prntiqne e t l'on peldl seule- mcnt de'crire l e sy.rtdme rle p0re.r en fonction d e qrrelqrrer Irnes d e .res cnrncle'- ristiqrrcs d'cnscmble qlri perrvent benrrcoup limiter In 7nes11r.e d a n s laqrrelle des so1ution.r CXdCteJ sont possibles.
Calculations of heat flow through building materials are based on the assump- tion that the materials involved are dry, and laboratory measurements to establish coefficients are a l n ~ o s t always made on a dry basis. However, moisture is almost always present in practice. T h e reason for this undesirable situation is not hard to find. T h e presence of water, in porous materials, under conditions of heat flow creates a complication which scientists and engineers have not yet been able to deal with in any rational way.
While many workers are interested in and are contributing to knowledge on the subject of combined heat and moisture flow, too few are concerned with an understanding of the general case. There may therefore be some benefit in offering for critical comment a review of n Canadian program of research and the philosophy which has developed concurrently with it.
A series of experiments was begun at the University of Saskntchewan in 1947 and continued at the Division of Building Research. These are still in progress and are likely to be continued for some time to come. N o attempt will be made to document this paper by reference to the work of other laboratories; such references may be found in the individual papers forming the basis for this re- view.
EARLY I N T E R E S T
Attention was first drawn to some of the complexities introduced by the pre- sence of moisture in heat transfer experiments carried out on sawdust and shavings at the University of Saskatchewan. These materials were for a time widely used as thermal insulation in wood frame dwelling construction. Though invariably tested in a dry condition they always contained at least some moisture in service and at times could be quite wet as a result of rain penetration or of condensation.
Attempts to measure the conductivity of materials in a moist state in con- ventional hot plate apparatus were unsuccessful, a result which no doubt would not have surprised more experienced workers. These experiences led t o an interest in the "balance sensitivity" of guarded hot plates. An estensive series of tests on the characteristics of guarded hot plates has since been carried out.
EXPERIMENTS O N C O M B I N E D FLOW I N CLOSED SYSTEMS
A closed hot plate was constructed so that the test samples were prevented from exchanging moisture with the laboratory atmosphere. It was found that this apparatus, once properly balanced, would remain so for weeks o n end, even when moist samples were used. T h e reasons for the earlier difficulties then became evident.
T h e closed guarded hot plate was then used in a series of experiments with moist sawdust [ I ] . T h e hot plate apparatus was limited in application, since it could only provide results of value under steady-state conditions. A second apparatus employing two hollow plates and a heat meter were used for con- tinuous observation of heat flow during the period of redistribution of contained moisture, originally uniformly distributed, to a moisture content pattern charac- teristic of the sample and the imposed temperature gradient. Solvason carried out further studies of a similar nature [2], with emphasis on transient conditions.
These experiments [I, 2 ) proved to be most significant, since they not only
indicated the next step in the program, but are still a source of stimu-
lation.
Briefly they established that:
1. Moisture migrates to the cold face.
2. The redistribution occurring can be predicted from known equilibrium
sorption data on the material so long as the maximum moisture content after redistribution nowhere exceeds the "fibre saturation point".
low conductivity material is srnall in the redistributed state, but substantial when the moisture is migrating.
4. At moisture contents above the fibre saturation point (30% by weight for wood) under a temperature gradient there appeared to be a capillary flow from the wet zone towards drier zones, and a corresponding but opposed vapour flow from dry (and warm) to wetter zones.
At the time these experiments were carried out, very few workers accepted the idea that part, at least, of the added heat transmission due to the presence of moisture might be d u e to the latent heat carried by the migrating vapour. Many recognized the migration of moisture to the cold side, but considered that this was mainly an inconvenience in determining "wet conductivities" for use in the conventional heat flow equation, and did not accept the possibility that under some conditions, at least, parallel flow mechanisms, highly interrelated, might be involved.
It was not until the con~pletion of new facilities in Ottawa in 1933 that further experiments were done using other materials such as moist soils. Such experi- ments carried out by Woodside and Cliffe [3] and Woodside and de Bruyn [4], demonstrated that the dense materials experienced marked redistribution under temperature gradients, if coarse grained, but not so marked if they were fine grained. This lent further weight to the view that the "strength" of the capillary system could resist some tendency of moisture to migrate to the cold side, a n d thus minimize the gradients in moisture contents produced in a closed system subjected to a temperature gradient.
It was concluded from these experiments that the influence of moisture upon heat transmission was related not only to the amount of moisture present but also to the rate of movement of nloisture under the existing temperature gra- dients. It was further concluded that transient methods designed to minimize the shift of moisture during a heat transmission measurement could not, in fact, avoid the influence of a moisture migration related to the particular condition of test. Woodside subsequently studied the use of the probe in moist materials [5] and concluded that vapour migration away from the probe during a test can influence significantly the heat transmission.
EXPERIMENTS O N T H E M E C H A N I S M O F MOISTURE M I G R A T I O N In the meantime Swenson, Sereda and Kuzmak carried o u t studies of the mechanism by which moisture was caused to migrate by a temperature gradient [ 6 , 7, 8 ) . These studies were carried out with fairly dense porous materials such as sand, at moderate to high percentages of saturation and led t o the conclusion that vapour flow was the mechanism by which moisture migration was produced; no thermally actuated flow of measurable quantity occurred in saturated m a - terial. However, once the material was unsaturated so as to produce voids, the temperature gradient across these pro,4-~ced vapour pressure differences and a corresponding transfer of moisture which in turn affected the cdpillary equili- brium and induced capillary flows as well. T h e factor producing flow was shown to be the vapour-filled gap or void. At this point Kuzmak began to contemplate, as had Inany others, the disparity between the rates of flow induced by tem- perature gradients and the calculated flow rates for vapour diffusion through unfilled pores. It was generally conceded that actual flow rates were usually from 4 to 1 0 times those predicted by calculation. Kuzmak and Woodside then proposed that these results could only be attributable to the existence of temperature gra- dients across unfilled pores which were greater than the average gradients. They constructed n ldrge scale rllodel o f a cubical pack of spheres a n d measured the temperatures at the void surfaces and found that these were indeed such as t o lead to temperature gradients much higher than average [9]. Other workers, Philip and de Vries, had in the meantime predicted that this should be true. A troublesome anomaly was thus dispelled and a further advance made in under- standing the nature of the mechanism or mechanisms involved.
DISCUSSION O F MECHANISM O F MOISTURE M I G R A T I O N
Vapour migration under isothermal conditions can be regarded as a special case of combined heat and moisture Flow and is worthy OF study in this con- nection. Actually true vapour diffusion occurs alone in-only a Few cases with actual materials. In most cases, the Flow coefficient to be used in a simple Flow equation based on Fick's L3w will be Found to be a Function OF relative humidity, increasing as relative humidity increases
[lo).
This occurs because OF the interac- tion between water lnolecules and the surface of the solid. W a t e r is adsorbed, Forming surface Films and may at higher relative humidities completely fill pores, the size of the pore which will fill being dependent mainly on relative llumidity. As soon as pores are Filled, capillary Flow, in series - parallel with vapour flow becomes possible. Strictly speaking, conditions are n o longer isothermal, when adsorption and Filling OF pores occurs, even tllough the bounding faces OF the material are held at constant temperature, nor is the Flow strictly a vapour flow even though water may enter and leave the material in the vapour state.Under isothermal conditions of vapour flow as described above the movement of liquid in Filled capillaries and in vapour filled pores are in the same direction, From the side OF high vapour pressure to that of low vapour pressure, and are additive in t e r m OF moisture migration. This need not always be so whell temperature gradients exist along with vapour pressure gradients. When tem- perature decreases in the direction of the imposed vapour flow condition, i. e., both vapour and temperature gradients have the same sign, the relative humidity and therefore the moisture contents may untler certain conditions increase in the direction of flow indicated by the vapour pressure gradient. This is so because relative humidity to which equilibrium nloisture contents are Inore or less uniquely related for a given material is given by the ratio of actual vapour pressure to the saturation pressure at the existing ternpera//cre. It is therefore possible that vapour migration which depends only o n vapour pressure may take place in one direction while Flow in the capillary system which is more or less directly related to relative humidity gradient, or to moisture content gradient, may tend to be in the opposite direction.
It was stated, in describing the work of Paxton with lnoist sawdust [ I ) that the situation appeared to be one of a n overall capillary flow opposing the vapour Flow. A similar situation is suggested above. Recalling however, the work of K~rzlnak and Sereda [7, 8 ) ant1 Kuzlnak and Woodside [3) which pointed to the telnperature gradients : ~ n d correspontling vapour pressure gradients across vapour-filled pores as the activating mechanism For flow, it will be realized that no continuous Flow in either vapour or liquid phase need take place. This may be possible in the case of s a w d i ~ s t which consists of sorbent particles in contact, surrounded by large voids, but in general it is appropriate to nssurne a more uniform gradation ant1 distribution of pore sizes, s o that small pores Filled with liquid rnay break the continuity of vapour Flow. T h e overall mechanism undoub- tedly involves a series - par:lllel arr~lngement of vapour-Filled and liquid-filled pores, with continuous evaporation and condensation occurring between liquid and vapour boundaries. T h e net result viewed macroscopically appedrs to be described by overall capillary and vapour flows.
C O N C L U S I O N
W h a t then, are the real problems in describing combined heat a n d moisture Flow quantitatively? They are First of all that the exact geometry of the pore systeln in sequence along the Flow path should be known in detail. T h i s is clearly impractical, if not impossible. T h e best that can be visualized is to be able to describe the pore system in terms of measurements of characteristics such as surface area, gas permeability, sorption isotherms for water vapour, and, at the high moisture contents, suction-versus-moisture-content relationships. I t is clear that in addition to a detailed knowledge of the pore geometry the completely analytical approach would also require a detailed theoretical knowledge of the
heat and moisture flow for the infinite number of situations presented by indi- vidual pores under a variety of temperature a n d moisture conditions.
T h e only hope for the d e v e l o p ~ n e n t of a quantitative approach lies in experi- ment, guided by tlie best possible understanding of w h a t goes on. Perhaps ways can be found by which measures of the characteristics of the p o r e system can be used to establish empirically riither than theoretically and o n an averaging rather than a spot basis the desired relationships for any particular material.
T h e appalling difficulty of dealing with c o ~ n b i n e d heat and moisture flow is countered only by the very great need to be able to deal with s ~ l c l l phenomena on a quantitative basis even t h o u g l ~ only approximately, in order to ncllieve advances in technology. Fortunately, as always in engineering, limited cases and ranges of conditions a r e likely to be more manageable t h a n tlie general case.
RIIFERENCES
1. PAXTON, J. A. and HUTCHEON, N. B. M o i s t ~ ~ r e &figration in a Closed GuartleJ Hot Plate. l'm~isuc~iorrs, At~ierirrrti Society of Hrdtitig a ~ i d T'e~itil~ii~tg E ~ i ~ i t i e e ~ s , 58, 301--)20, 1952.
2. SOLVASON, K. K. Moisture in Transient Heat Flow. Ibitf., 62, 1 1 1-122, 1956. 3. WOODSIDE, w. and CLIFFE, J . B. Heat and Moisture Transfer in Closed Systcnis of Two Granular Materials. Soil Scie~ice, 87, ( 2 ) , 75-82, Feb. 1959.
4. WOODSIDE, w. and DE U I ~ I I Y N , (:. bf. A . Hcat Transfer- in a Moist Clay. Soil Si.ietir.r,
87, ( 3 ) , 16G173, Mar. 1959.
5. ~VOODSIDI.:, w. Probe for l'hcrrnal Con~luctivity Measurement u F Dry ;~ntl Moist hiatc- rials. Hetrti~ig Pipi~ig snd Air Co~tdiiioni~rg, 30, 163-170. Sept. 1958.
6. SWENSON, E. G. ant1 SERI.:DA, P . J. I'reli~ninary Experiments on tlie Movcment of W.tter Through Concrete and Other Materials Due to a Temperature Gradient. R ~ i l l . N o . 1. Division of Builtling Research, National Research Council (Canatla), (3568), 102-109,
1955.
7. KUZkLAK, J. hl. and Sl<REDA, P. J . ?'he h.iechanism by whicii Water Moves 'lhrough :I 130rous Mgiterial Subjected to a Te~nperature Gradient. 2 . Salt tracer and streil~ning po-
into a Si~turated System. Soil Scirticr, 84, ( i f ) , 291-299, Oct. 19j7.
8. KUZMAK, J. hi. and SEBEDA, P. J. Tlic Meclianisni by which Water Moves Through
:I Porous Material Subjected to a te~npcrature Gradient. 2. Salt tracer and strelrming po-
tential to detect flow and in tlie liquid phase. Ibid. 84, 119-422, Nov. 1957.
9. ~ ~ O D S I D E , w. and KUZAIAK, J . ht. Effect of Temperature Distribution on Moisture Flow in Porous Idateriais. ?'rrrtisrri.iio~r.r, At~in.icrr~t G e o p l ~ ~ s i c ~ l 1.J1rio11, 39, (4), 676-6SO.
Aug. 195s.
10. CI-IANG, S. c:. ;lntl IILJ.I.C:~IEON, N . o. Depenclenci- o f LV;ltcr Vapour Perlnmbility on ? <
1 emperature ancl Humiclity. 'L'r~~is;rr/ioni, ,.I l?irrir.a~t Sucirly of Ile;riitig n ~ d A i r - C o ~ ~ d i -
![[PDF] Architecture J2EE support de cours complet de A a Z | Cours j2ee](data:image/gif;base64,R0lGODlhAQABAIAAAP///wAAACH5BAEAAAAALAAAAAABAAEAAAICRAEAOw==)