1、 International Journal of Heating, Ventilating, Air-conditioning and Refrigerating Research Editors John W. Mitchell, Ph.D., P.E. Professor of Mechanical Engineering University of Wisconsin-Madison, USA Associate Editors James E. Braun, Ph.D., P.E., Associate Professor, Ray W. Hemck Laboratories, Ar
2、thur L. Dexter, D.Phil., C.Eng., Reader in Engineering Science, Department of Leon R. Giicksman, Ph.D., Professor, Departments of Architecture and Ralph Goldman, Ph.D., Senior Consultant, Arthur D. Little, Inc., USA Hugo Hens, Dr.Ir., Professor, Department of Civil Engineering, Laboratory of Buildin
3、g Physics, Katholieke Universiteit, Belgium Anthony M. Jacobi, Ph.D. Associate Professor and Associate Director ACRC, Department of Mechanical and Industrial Engineering, University of Illinois, Urbana-Champaign, USA Ken-Ichi Kimura, Dr. Eng., Professor, Department of Architecture, Waseda University
4、 and President, Society of Heating, Air-conditioning and Sanitary Engineers of Japan, Japan Angewandte Wrmetechnik, Universitt Hannover, Germany Universit de Lige, Belgium Engineering, Department of Mechanical Engineering, University of Maryland, College Park, USA School of Mechanical Engineering, P
5、urdue University, West Lafayette, Indiana, USA Engineering Science, University of Oxford, United Kingdom Mechanical Engineering, Massachusetts Institute of Technology, Cambridge, USA Horst Kruse, Dr.-Ing., Professor Emeritus, Institut fr Kltetechnik und Jean J. Lebrun, Ph.D., Professor, Laboratoire
6、de Thermodynamique. Reinhard Radermacher, Ph.D., Professor and Director, Center for Environmental Energy Policy Committee Lynn G. Bellenger, chair Mario Costantino Hans O. Spauschus John W. Mitchell Frank M. Coda W. Stephen Comstock Editorial Assistant Jennifer A. Haukohl Publisher W. Stephen Comsto
7、ck ASHRAE Editorial and Publishing Services Staff Robert A. Parsons, Handbook Editor Scott A. Zeh, Publishing Services Manager Nancy F. Thysell, Typographer QI999 bv the American Societv of Heatinn. Refriaeratina and Air-Con- transmitted in any form or by any meanoelectronic. photocopying, ditioning
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12、ith any statement or opinion in this publica- tion. The entire risk of the use of any information in this publication is assumed by the user. Postmaster-Send form 3579 to: HVAC and (2) calculate the theoretical reduction in heat exchanger length that could be achieved with such grouts. Other grout p
13、roperties such as coeffi- cient of permeability, durability, shrinkage, heat of hydration, bonding and environmental impact are the subject of ongoing experimental characterization (Allan 1997, Allan and Philip- pacopoulos 1998). EXPERIMENTAL PROCEDURE Two strategies to increase the thermal conducti
14、vity of cementitious grouts were used simulta- neously. The first of these was to incorporate high thermal conductivity fillers in the grout for- mulations. The second strategy was to use a superplasticizer to enable reduction of the water content of the grout mix. Superplasticizer is a liquid addit
15、ive commonly used in the concrete industry to improve the rheological properties, reduce waterkement ratio and enhance durabil- ity. Lowering the waterkement ratio of grout through use of a superplasticizer decreases the porosity of the hardened material. This results in higher thermal conductivity
16、and improves other physical and mechanical properties. Materials Type I cement (ordinary Portland, ASTM C 150) was used, although Type II (moderate) or V (sulfate resistant) would be recommended for high sulfate environments. Fly ash and ground gran- ulated blast furnace slag were used as partial ce
17、ment replacements in some of the grout formula- tions. These supplementary cementing materials are recognized for their ability to enhance durability in adverse environments (e.g., aggressive groundwater), reduce heat of hydration and reduce cost. The fly ash conformed to ASTM C 61 8 Class F. This i
18、s a low calcium fly ash produced from combustion of bituminous coal. The blast furnace slag was ASTM C 989 Grade 100. Cement replacement levels of 40,60 and 75% by mass were used. The combined cement and fly ash or slag is referred to as cementitious material. The superplasticizer used was a sulfona
19、ted naphthalene type with a solids content of 42% by mass. Different particulate fillers were investigated for improving the thermal conductivity of cementitious grouts. The fillers were selected for their thermal conductivity and compatibility with producing a fluid grout. The materials included si
20、lica sand, alumina grit, silicon carbide grit, and steel grit. The proportion of filler was controlled to produce grouts with similar flowability. The fillers had different particle sizes and shapes. Hence, the mass and volume frac- tions of fillers varied between mixes. A small proportion of benton
21、ite was added to the cementitious grouts to reduce bleeding, pro- mote full-volume set, and improve filler carrying capacity (i.e., reduce settling). Bleeding occurs when a layer of water forms at the surface of the freshly placed grout due to segregation of the solids. Excessive bleeding could give
22、 rise to a weaker, more permeable surface layer of grout. Bleeding is discussed further by Neville (1996). The waterkementitious material ratio (w/c) of the filled grouts was kept constant at 0.45 (by mass) in this study so that the effect of filler on conductivity could be determined. The exception
23、 to this was a cement-sand grout without superplasticizer that had a waterkementitious material ratio of 0.75. Conventional neat cement (cement plus water) grouts and a cement-bentonite grout were tested to establish a baseline. These grouts did not contain any fillers. A bentonite-sand grout was al
24、so tested for comparison. The mix proportions of the tested grouts are given in Table 1. The proportions are by mass. Grouts containing either fly ash or blast furnace slag used partial replacement of cement by mass proportions of 40, 60, or 75%. VOLUME 5, NUMBER 2, APRIL 1999 89 Table 1. Mix Propor
25、tions by Mass of Grouts Studied (w/c = watedcement ratio; f/c = filledcement ratio) % % % Super- Description % Water Cement Bentonite plasticizer % Filler w/c f/c Neat cement + 27.9 69.8 0.6 1.7 0.4 superplasticizer Neat cement 37.2 62.0 0.8 0.6 Neat cement 44.0 55.1 0.9 0.8 Cement + bentonite 76.0
26、18.0 6.0 4.2 Cement + sand 16.7 37.1 0.8 0.9 44.5 0.45 1.2 Cement + sand 12.9 28.6 0.6 0.7 57.2 0.45 2.0 Cement + sand 19.9 26.5 0.6 53.0 0.75 2.0 Cement + coarse 13.3 29.7 0.7 0.7 55.6 0.45 1.9 alumina Cement + fine 16.0 35.5 0.3 0.8 47.4 0.45 1.3 alumina Cement + coarse 15.4 34.1 0.8 0.8 48.9 0.45
27、 1.4 silicon carbide Cement + fine silicon 17.3 38.5 0.9 0.9 42.4 0.45 1.2 carbide Cement + steel grit 8.4 i 8.7 0.6 0.5 71.8 0.45 3.8 Specimen Preparation The cementitious grouts were cast as blocks 75 mm x 125 mm x 25 mm (3 in. x 5 in. x 1 in.). Three specimens per batch were cast. The blocks were
28、 sealed, demolded after 24 hours, and placed in a water bath to cure. The hardened grouts were tested for thermal conductivity at an age of 14 days. The grouts were then dried in an oven at 40C (104F) over a period of seven days, allowed to cool, and re-tested to determine the effect of loss of mois
29、ture. This was not intended to completely dry the grout. The value of 40C was chosen to reflect a typical maxi- mum temperature to which the grout would be exposed in service. Specimens were weighed to determine percentage mass loss on drying. Selected grouts were re-saturated after drying and therm
30、al conductivity was re-measured. The bentonite-sand grout was cast as blocks with the same dimensions as the cementitious grouts. The specimens were sealed initially to prevent loss of moisture. Thermal conductivity was measured at four and 24 hours. The specimens were then allowed to dry in air at
31、21C (70F) and relative humidity of 40 to 50% for 14 days. Thermal Conductivity Measurements Thermal conductivity of the grouts was measured using a thermal conductivity meter that uses the hot wire method to calculate the thermal conductivity h. The hot wire test is a transient method and therefore
32、overcomes the problem of moisture migration and subsequent decrease in thermal conductivity of moist grouts that would occur with a steady state method. A probe con- sisting of a heater and thermocouple on the surface of a sole plate with known thermal conduc- tivity is placed on the surface of the
33、material to be tested. Constant current is passed through the heater wire and the electromotive force of the thermocouple at the time is automatically recorded. The grout specimens were heated from an initial temperature of 21C to 60C (70F to 140F) over one minute. The instrument calculated and disp
34、layed the thermal conductivity in 90 HVAC f/c = filler/cement ratio) Thermal Conductivity, W/(m.K) % Mass Filler Material wlc f/C Saturated After Drvine Loss Silica sand Silica sand Silica sand Coarse alumina Fine alumina Coarse silicon carbide Fine silicon carbide Steel grit 0.45 0.45 0.75 0.45 0.4
35、5 0.45 0.45 0.45 1.2 2.0 2.0 1.9 1.3 1.4 1.1 3.8 1.73 1 f 0.03 1 2.394 f 0.045 2.161 r0.042 2.318 f 0.030 2.075 f 0.036 3.302 f 0.056 2.725 f 0.039 2.895 0.049 1.605 f 0.037 2.265 f 0.043 1.491 f 0.048 2.128 f 0.039 1.794 f 0.028 2.493 f 0.043 2.147 f 0.031 1.920 f 0.027 3.3 1.7 7.7 2.9 3.2 2.9 3.2
36、3.6 Table 4. Thermal Conductivity of Bentonite-Sand Grout Age/Condition Thermal Conductivity, W/(mK) % Mass Loss 4 hours/moist 1.462 f 0.020 - 24 hourslmoist 1.423 f 0.021 - 2 days/air dried 1.342 f 0.027 13.8 7 days/air dried 0.658 f 0.031 32.0 14 days/air dried 0.502 f 0.028 33.7 VOLUME 5, NUMBER
37、2, APRIL 1999 91 DISCUSSION Thermal Conductivity Neat Cement and Cement-Bentonite Grouts The thermal conductivity of the cement-bentonite grout was the lowest of all the cementitious materials tested in the saturated state, thereby suggesting unsuitability for ground source heat pump applications. T
38、he neat cement grouts also had relatively low thermal conductivities. The effect of waterkement ratio (wk) is demonstrated by comparing the results for grouts with ratios of 0.4, 0.6, 0.8, and 4.2 in Table 2. Thermal conductivity increases with decreasing w/c. When the amount of water in the origina
39、l mix exceeds that required for hydration of cement the excess can be evaporated, thus leaving pores in the hardened grout. These pores act to decrease thermal conductivity. The results for the neat cement grouts can be compared with data for cement pastes at differ- ent waterkement ratios. Under sa
40、turated conditions thermal conductivities of 1.3, 1.2, and 1 .O W/(m.K) for waterkement ratios of 0.4, 0.5, and 0.6, respectively have been reported by Mind- ess and Young (1981). The tested grouts with w/c of 0.4 and 0.6 had lower values. Without fur- ther information on the specimen preparation, a
41、ge and testing conditions for the cement paste data from Mindess and Young it is not clear why the tested neat cement grouts had lower ther- mal conductivities. Retention of thermally conductive properties under dry conditions is an important require- ment. Loss of moisture in the media surrounding
42、the heat exchanger occurs as a result of heat rejection when a ground source heat pump operates in the cooling mode. The oven drying tests give an indication of material performance under aggressive drying conditions. The cement-ben- tonite grout exhibited cracking and friability after oven drying a
43、nd would act as an insulator if similar dehydration occurred during exposure. The neat cement grout with waterkement of 0.8 showed a significant decrease in mean thermal conductivity of 43.2% on oven drying. Compari- son with the superplasticized grout with waterkement of 0.4 demonstrated that the p
44、ercentage decrease in thermal conductivity on drying was reduced to 18.7% by lowering w/c. Heat transfer studies by Braud (1991) and Braud and McNamara (1989) have shown that neat cement grout performs similarly to commercial bentonite grouts under the test conditions used. This behavior is in agree
45、ment with the relatively low thermal conductivity measured on neat cement grouts Cementitious Grouts Containing Fillers The results in Table 3 show that addition of suitable filler materials to cementitious grouts can improve thermal conductivity significantly. Fillers also reduce shrinkage and crac
46、king. Due to the different particle shapes and sizes of the filler materials, the proportion of filler could not be kept constant while retaining the same flow properties. Thus, the conductivity results are not solely a reflection of the filler composition, but also of the amount of filler that coul
47、d be added. Further improvement in thermal conductivity may be possible, particularly through manipula- tion to improve packing, volume fraction and contact between particles while maintaining grout pumpability. The importance of maximizing filler content to improve thermal conductivity is illustrat
48、ed in Figure 1. The grout was a 40% slag/60% cement mix and the sand grading was the same for all mixes. The graph shows a linear relationship between thermal conductivity and sandkernenti- tious material ratio. Therefore, it is possible to tailor the thermal conductivity to meet require- ments thro
49、ugh control of the sand content. 92 HVAC hd - 7.69 W/(m.K) (4.44 Btu/h.ft.“F) for quartz (Gueguen and Palciauskas 1994); and Vd = 0.501. Equation (1) predicts a thermal conductivity of 2.678 W/(m.K) (1.548 Btu/h.ft.“F), which compares with the measured value of 2.394 Wi(m.K) (1.384 Btu/h.ft.“F). At a s/c of 1.2 (Vd= 0.375) the predicted value is 1.966 W/(m.K) (1.136 Btu/h.ft.“F) compared with the experimental value of 1.731 W/(m.K) (1.000 Btu/h.ft.“F). The difference between experimental and predicted values can be attributed to capillary
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