阅读说明：本技术 地源热泵系统施工中钻孔回填材料的导热系数测定方法 (Method for measuring heat conductivity coefficient of drill hole backfill material in ground source heat pump system construction ) 是由 邓军涛 郑建国 张继文 王娟娟 张丽娜 于 2019-10-14 设计创作，主要内容包括：本发明公开了地源热泵系统施工中钻孔回填材料的导热系数测定方法,涉及地源热泵技术领域,在室内测定组成回填材料的各种材料的导热系数,并测定多种不同配比的回填材料的导热系数,在现场测定回填至钻孔内的回填材料的导热系数,利用现场测得的导热系数对室内测得的导热系数进行修正。本发明采用室内和现场测定相结合的方式,大大提高了回填材料导热系数测定的准确性。(The invention discloses a method for measuring the heat conductivity coefficient of a backfill material of a drill hole in the construction of a ground source heat pump system, which relates to the technical field of ground source heat pumps. The invention adopts a mode of combining indoor and field measurement, thereby greatly improving the accuracy of measuring the heat conductivity coefficient of the backfill material.)

1. The method for measuring the heat conductivity coefficient of the backfill material of the drill hole in the construction of the ground source heat pump system is characterized by comprising an indoor test stage and a field test stage, wherein in the indoor test stage, a heat conductivity coefficient tester is used for measuring the heat conductivity coefficients of bentonite, cement and sand, and backfill materials with various proportions are selected for measuring the heat conductivity coefficients;

in the field test stage, the backfill materials are respectively proportioned in the backfill chambers around the buried pipe in the same field, the same heat exchange pipe diameter and the same heat exchanger form in the loess area, and the thermal conductivity coefficients of different backfill materials are measured by utilizing a rock-soil thermophysical property tester;

and after the heat conductivity coefficient is obtained through field test determination, the heat conductivity coefficient obtained through indoor test determination is corrected by using the measured heat conductivity coefficient.

2. The method for measuring the thermal conductivity of the backfill material drilled in the ground source heat pump system construction as claimed in claim 1, wherein in the indoor test stage, the backfill material and the sample of each material composing the backfill material are molded by a triple die, the sample is demolded after 24 hours of molding, different samples are numbered and placed in a curing room for curing, and the thermal conductivity test is carried out after 2-3 days of curing.

3. The method for measuring the thermal conductivity of the backfill material drilled in the ground source heat pump system construction as claimed in claim 1, wherein the thermal conductivity of the bentonite is measured by placing the bentonite into a fixed container and measuring the thermal conductivity after the completion of the shrinkage of the bentonite;

when the heat conductivity coefficient of the sand is measured, the sand is placed in a fixed container, samples are prepared by adopting different dry densities and different water contents, and the heat conductivity coefficient is measured.

4. The method for measuring the thermal conductivity of the backfill material for the borehole in the ground source heat pump system construction as claimed in claim 1, wherein the cement, the sand and the water in the backfill material are proportioned according to the water cement ratio of 0.55 and the sand cement ratio of 2.0, and a certain amount of bentonite is added to prepare a sample.

5. The method for measuring the thermal conductivity of the backfill material drilled in the ground source heat pump system construction process according to claim 1, wherein the geotechnical thermal property tester comprises an electric heater, a heat preservation water tank, a water pump, a pipeline system, a power distribution system, a flow sensor, a temperature sensor, a power transmitter and a data acquisition device, and the components are all integrated on a fixed rack.

Technical Field

The invention relates to the technical field of ground source heat pumps, in particular to a method for measuring the heat conductivity coefficient of a drill hole backfill material in the construction of a ground source heat pump system.

Background

In the design and construction process of the buried pipe ground source heat pump, the backfill material is required to have good pumpability, so that the U-shaped pipe can form good contact with surrounding rock and soil mass, and the contact thermal resistance is reduced. Over 90% of heat exchange holes in loess areas are mostly backfilled by adopting primary pulp generated by drilling, and additives such as sand, cement, quartz, a water reducing agent and the like are added into backfilling materials in individual fields so as to improve the heat conductivity and compactness of the backfilling materials. The backfill reason of the primary pulp mainly comprises the following three points: reducing the engineering quantity of outward transportation of slurry; secondly, the construction is convenient and fast, and the heat exchange holes are communicated by digging grooves; and thirdly, the compactness of the backfill around the buried pipe can be ensured (the compactness is almost close to that of the original soil after a period of time of precipitation after the primary pulp is backfilled). The construction mode can also adopt a mechanical grouting mode for backfilling, namely, prepared backfill material fluid is filled around the buried pipe through a grouting pump.

During the construction of the drilling machine, bentonite is needed to make a slurry protection wall for ensuring the stability of the wall of the heat exchange hole, and the content of the bentonite in the drilling process is generally determined according to the condition of a drilling stratum. As known from the drilling process, the backfilled raw stock mainly comprises a mixed material of bentonite, water and a drilling substance. In the field test stage, no enough heat exchange holes are provided for providing primary pulp, so outsourcing sand materials are adopted for backfilling. Therefore, it is necessary to measure the difference of the heat conductivity of different backfill materials, so as to optimize the backfill materials of the geothermal energy exchange system in the loess area. However, there is currently no method for accurately determining the thermal conductivity of backfill materials.

Disclosure of Invention

The embodiment of the invention provides a method for measuring the heat conductivity coefficient of a drill hole backfill material in the construction of a ground source heat pump system, which can solve the problems in the prior art.

The invention provides a method for measuring the heat conductivity coefficient of a drill hole backfill material in the construction of a ground source heat pump system, which comprises an indoor test stage and a field test stage, wherein in the indoor test stage, a heat conductivity coefficient tester is used for measuring the heat conductivity coefficients of bentonite, cement and sand, and backfill materials with various proportions are selected for measuring the heat conductivity coefficients;

in the field test stage, the backfill materials are respectively proportioned in the backfill chambers around the buried pipe in the same field, the same heat exchange pipe diameter and the same heat exchanger form in the loess area, and the thermal conductivity coefficients of different backfill materials are measured by utilizing a rock-soil thermophysical property tester;

and after the heat conductivity coefficient is obtained through field test determination, the heat conductivity coefficient obtained through indoor test determination is corrected by using the measured heat conductivity coefficient.

The method for measuring the heat conductivity coefficient of the backfill material of the drill hole in the ground source heat pump system construction comprises the steps of measuring the heat conductivity coefficients of various materials forming the backfill material indoors, measuring the heat conductivity coefficients of the backfill materials with different proportions, measuring the heat conductivity coefficient of the backfill material backfilled into the drill hole on site, and correcting the heat conductivity coefficient measured indoors by using the heat conductivity coefficient measured on site. The invention adopts a mode of combining indoor and field measurement, thereby greatly improving the accuracy of measuring the heat conductivity coefficient of the backfill material.

Drawings

In order to more clearly illustrate the embodiments of the present invention or the technical solutions in the prior art, the drawings used in the description of the embodiments or the prior art will be briefly described below, it is obvious that the drawings in the following description are only some embodiments of the present invention, and for those skilled in the art, other drawings can be obtained according to the drawings without creative efforts.

FIG. 1 is an installation schematic diagram of a rock-soil thermophysical property tester;

FIG. 2 is a schematic diagram showing the variation of thermal conductivity of bentonite with dry density;

FIG. 3 is a plot of heat conductivity of sand as a function of water content;

FIG. 4 is a graph of the thermal conductivity of sand as a function of dry density;

FIG. 5 is a backfill material flow curve.

Detailed Description

The technical solutions in the embodiments of the present invention will be clearly and completely described below with reference to the drawings in the embodiments of the present invention, and it is obvious that the described embodiments are only a part of the embodiments of the present invention, and not all of the embodiments. All other embodiments, which can be derived by a person skilled in the art from the embodiments given herein without making any creative effort, shall fall within the protection scope of the present invention.

The invention provides a method for measuring the heat conductivity coefficient of a drill hole backfill material in the construction of a ground source heat pump system. In the indoor test stage, heat conductivity coefficients of bentonite, cement and sand are measured, and backfill materials with various proportions are selected for heat conductivity coefficient measurement. In the indoor test, the backfill material and the sample of each material forming the backfill material are molded by a triple die with the inner wall dimension of 40mm multiplied by 160mm, and the die is removed after 24 hours. And numbering different samples, placing the samples into a curing room for curing, and carrying out heat conductivity coefficient test work after curing for 2-3 days. Placing bentonite into a fixed container, and measuring the thermal conductivity after the bentonite shrinks; the sand was first placed in a fixed container using different dry densities (1.2 g/cm)³、1.3g/cm³、1.4g/cm³、1.5g/cm³、1.7g/cm³) Samples were prepared at different water contents (10%, 15%, 20%, 25%, saturated) and the thermal conductivity was measured. The cement, sand and water in the backfill material are proportioned according to the water-cement ratio of 0.55 to 2.0, and on the basis, a proper amount of bentonite (0.2%, 0.5%, 0.7%, 1.0%, 1.2% and 1.5%) is added to prepare a sample, so that the heat conductivity coefficients of the backfill materials with different proportions are measured.

In the field test stage, materials matched in a backfill chamber around a buried pipe are respectively selected in the same field and in the same heat exchange pipe diameter (32mm) and heat exchanger form (double U) in the loess area, so that the heat conductivity coefficients of different backfill materials are measured. After the heat conductivity coefficient is obtained through field test measurement, the heat conductivity coefficient obtained through indoor test measurement is corrected by using the heat conductivity coefficient.

The tester mainly comprises an electric heater, a heat-preservation water tank, a water pump, a pipeline system, a power distribution system, a flow sensor, a temperature sensor, a power transmitter, a data acquisition device and the like, is integrated on a fixed rack, the heat-preservation water tank is used for storing a medium heated by the electric heater, the pipeline system is used for inserting into a drill hole to enable the medium to exchange heat with the ground, the power distribution system is used for supplying power, the power transmitter is used for measuring power supply power, the data acquisition device is used for acquiring and storing data acquired by the flow sensor and the temperature sensor, and the tester is installed as shown in figure 1.

The method and effects of the present invention will be described below with reference to specific experiments.

Measurement of thermal conductivity of Bentonite

The bentonite has long-term physicochemical stability, low permeability and nuclide adsorption, can be applied to high-radioactivity waste treatment, and meanwhile, the bentonite has excellent wall-fixing capacity and is widely applied in the drilling process. In the early stage, the design and construction of the ground source heat pump of the buried pipe also adopts the mixture of bentonite and water as a backfill material, and the backfill material is gradually abandoned later due to the reasons of smaller heat conductivity coefficient, easy drying shrinkage, dry cracking and the like.

The test adopts 200-mesh kolkali bentonite, which consists of water-containing aluminosilicate minerals and mainly comprises SiO as a chemical component₂、Al₂O₃And H₂And O. In addition, some Fe is contained₂O₃、MgO、CaO、Na₂O, and the like. The curve of the thermal conductivity of bentonite with respect to dry density at a water content of 20% is shown in FIG. 2.

The result of measuring the thermal conductivity of the bentonite shows that the thermal conductivity of the bentonite tends to increase gradually with the increase of the dry density, and when the thermal conductivity reaches 0.8W/(m.k), the thermal conductivity of the bentonite changes less with the increase of the dry density and does not increase basically. The variation range of the thermal conductivity coefficient of the bentonite with different dry densities is 0.45-0.9W/(m.k). Van Ching measures the thermal conductivity of the bentonite molding material with initial water content of 3.5%, and the result is 0.232W/(m.k). It follows that a single bentonite clay can not be used as the heat exchanger tube periphery backfill material.

Determination of thermal conductivity of cement

PO32.5 and PO42.5 type cements were used. The sample was prepared using a water-cement ratio of 0.55, and the results of the measured thermal conductivity are shown in table 1.

TABLE 1 Heat conductivity coefficients of different types of cement

Type of cement	PO32.5	PO42.5
			Thermal conductivity W/(m.k)	0.91	1.03

As shown in Table 1, the thermal conductivity of PO42.5 cement is slightly higher than that of PO32.5 because the PO42.5 has higher strength and the cement mortar is denser.

Determination of Heat conductivity of Sand

The sand belongs to a discrete material and is difficult to shape, the sand is placed in a fixed container, water distribution is carried out after the dry density of the sand is controlled, and the heat conductivity coefficient of the sand is measured. After data with large differences are removed, the change curves of the sandy soil with different dry densities along with the water content are obtained, as shown in fig. 3.

As can be seen from fig. 3 and 4, as the water content and dry density increase, the thermal conductivity of the sand increases. Under the same water content, the heat conductivity coefficients of the sands with different dry densities have little difference. The influence of the water content on the heat conductivity coefficient of the sand is more obvious than the influence of the dry density. The heat conductivity coefficient of the dry sand is small, about 0.2W/(m.k), the heat conductivity coefficient of the sand increases along with the increase of the water content, the amplification is maximum when the water content is 0-10%, and the heat conductivity coefficient of the sand increases less along with the change of the water content after the water content reaches 20% (the sand is close to saturation).

Determination of heat conductivity coefficients of backfill materials with different proportions

During the construction of the buried pipe, the wall is generally protected by bentonite slurry in the drilled hole, and the buried pipe is buried after the slurry is changed by clear water after the drilling is finished to reduce the consistency of substances in the hole. After the slurry is replaced by clear water, most of the residual substances in the drill hole are the mixture of bentonite and water. The mixture has low heat conductivity coefficient, is easy to dry, shrink and crack so as to be separated from the U-shaped pipe to form a large gap, and seriously influences the heat conduction performance of the heat exchange hole. Reference values for thermal conductivity for various backfill materials of different composition ratios are given below and are listed in table 2.

TABLE 2 thermal conductivity of backfill materials

As shown in Table 2, the higher the sand content, the higher the thermal conductivity of the backfill material, which can reach a maximum of 2.42. The existing research results show that the flowing performance of the backfill material is the best when the water-cement ratio is 0.55-0.6 and the sand-cement ratio is 2.0. On the premise of ensuring the flowing performance of the backfill material, auxiliary additives can be added properly to obtain a proper heat conductivity coefficient of the backfill material. When the heat conductivity coefficient of the backfill material is lower than that of the rock and soil mass around the buried pipe, heat transfer in the hole forms a heat exchange bottleneck, which is not beneficial to heat exchange of the buried pipe; when the heat conductivity coefficient of the backfill material is higher than that of the rock-soil mass around the buried pipe, the heat exchange capability in the holes of the buried pipe is enhanced, but heat transfer is carried out among the branch pipes, so that the thermal short circuit phenomenon is generated. Therefore, the heat conductivity coefficient of the backfill material is only consistent with or slightly higher than that of the rock-soil mass around the pipe. The heat conductivity coefficient of the rock-soil mass in the loess area is 1.065-2.3W/(m.k), and a proper material can be selected in the interval without a superconducting material with too high heat conductivity coefficient. Based on this, the two mixed materials (bentonite + water + sand + cement, bentonite + water + sand) were subjected to an indoor test measurement of the thermal conductivity.

The bentonite is added into the material with sand and cement as main base material to raise the slurry stability and reduce sand separation. The flow properties of the material were measured at 3%, 2.5%, 2%, 1.5%, 1.0%, and 0.5% of the added amount of bentonite, and the results are shown in fig. 5.

According to fig. 5, the fluidity of the slurry becomes worse as the content of bentonite increases. When the addition amount of the bentonite is close to 3 percent, the backfill material has the phenomena of difficult mixing, water loss cracking and the like, and the fluidity is reduced to 150 mm. When the addition amount of the bentonite is less than 1%, the fluidity of the bentonite is greatly improved. Therefore, the amount of bentonite added was determined to be 1%.

The water-cement ratio, the sand-cement ratio and the content of bentonite in the backfill material have great influence on the heat conductivity coefficient of the backfill material, and other addition materials such as a water reducing agent, an expanding agent and the like mainly aim to improve the flowing property and the expansion property of the backfill material, do not greatly contribute to the heat conductivity coefficient of the backfill material, and can be added in a proper amount in the actual construction process. According to the existing research results, the addition amount of the optimal water reducing agent is 1.4%, and the addition amount of the expanding agent is 6% which is more appropriate; some documents recommend 1.5% water reducing agent and 5% bulking agent. According to the test result, the addition amounts of the water reducing agent are 1.4% and 1.5%, and the addition amounts of the expanding agent are 5% and 6%, so that the test result is not greatly influenced, but when the addition amounts of the water reducing agent and the expanding agent are continuously increased, the flowing and expanding effects of the backfill material are not remarkably changed any more.

1. Determination of heat conductivity coefficient of backfill material consisting of bentonite, water and sand

According to the above, in order to maintain the better fluidity of the backfill material, three samples were prepared with 1% bentonite content, 0.55 water-cement ratio and 20% sandy soil water content, and the test results are shown in table 3.

TABLE 3 summary of thermal conductivity test results

Sample numbering	Thermal conductivity (W/(m.k))
		1	1.18
2	1.23
		3	1.20

As can be seen from Table 3, the thermal conductivity of the backfill material prepared by mixing bentonite, water and sand is at least 1.20W/(m.k), and compared with that of sand with 20% water content in FIG. 3, the thermal conductivity is 1.04-1.11W/(m.k), and after the bentonite is added, the thermal conductivity is increased by 6% -10% and is not increased greatly. Therefore, the bentonite has low self-thermal conductivity and small contribution to the increase of the overall thermal conductivity of the mixed material.

2. Determination of heat conductivity coefficient of backfill material consisting of bentonite, water, sand and cement

Three sets of samples were prepared with a ratio of water to cement to sand to bentonite of 0.55:1:2:0.01 and the results are shown in table 4 below.

Table 4 summary of thermal conductivity test results

Sample numbering	Thermal conductivity (W/(m.k))
		1	1.65
2	1.58
		3	1.55

As can be seen from Table 4, the sample prepared by mixing bentonite, water, sand and cement has a minimum thermal conductivity of 1.55W/(m.k), and compared with Table 3, the thermal conductivity is increased by about 30% compared with the sample prepared by mixing bentonite, water and sand. The reason is that the cement as a powdery hydraulic inorganic cementing material can be better hardened in water, sand grains are firmly cemented together, the compactness of a sample is greatly enhanced, and the heat conductivity coefficient of the sample is greatly increased.

While preferred embodiments of the present invention have been described, additional variations and modifications in those embodiments may occur to those skilled in the art once they learn of the basic inventive concepts. Therefore, it is intended that the appended claims be interpreted as including preferred embodiments and all such alterations and modifications as fall within the scope of the invention.

It will be apparent to those skilled in the art that various changes and modifications may be made in the present invention without departing from the spirit and scope of the invention. Thus, if such modifications and variations of the present invention fall within the scope of the claims of the present invention and their equivalents, the present invention is also intended to include such modifications and variations.