CN115290693A - Improved method for measuring heat convection coefficient outside micro-fine tube based on double working media - Google Patents
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Abstract
The invention discloses an improved method for measuring the convection heat transfer coefficient outside a micro-tube based on double working media, which comprises the following steps: measuring physical parameters of working medium in the pipe, air outside the pipe and a convective heat transfer environment; calculating the qualitative parameter T inside the tube f 、P f (ii) a Calculating the physical property parameters of the working medium at the inner side of the tube according to the qualitative parameters in the tube; calculating the flow area A in the pipe fmedium And total heat transfer area A outside the tube hair (ii) a Calculating the flow velocity V in the pipe medium And Reynolds number Re medium (ii) a According to Dittus-Boelter empirical relation, calculating the convective heat transfer coefficient h in the tube in (ii) a Calculating logarithmic heat transfer temperature difference delta T according to inlet and outlet temperatures of air and working medium lm (ii) a = calculation of air Heat exchange quantity Q at Heat balance 2 Heat exchange quantity Q of working medium in tube 1 The heat exchange quantity Q; calculating a total heat transfer coefficient K based on the total heat transfer area outside the pipe; according to the heat transfer balance equation inside and outside the tube, the heat convection coefficient h of the air side outside the tube is calculated and obtained out . The method converts the measurement of the convective heat transfer coefficient into the solution of the thermal resistance; has high feasibility in theory and practice.
Description
Technical Field
The invention belongs to the technical field of heat exchange of efficient compact heat exchangers, and particularly relates to an improved method for measuring a heat exchange coefficient of micro-fine tube external convection based on double working media.
Background
The convective heat transfer coefficient is an intuitive parameter embodiment reflecting the strength of the heat exchange system, and is a comprehensive embodiment of the transmission property and the flow form of the medium. With the development of aircraft engines, the turbine front temperature and thrust-weight ratio are continuously increased, which puts higher requirements on cooling capacity, and thus, a high-efficiency compact micro-scale heat exchanger is introduced. However, the conventional empirical formula is mostly suitable for the calculation of the heat convection coefficient outside the micro-tube with a larger difference from the true value. Therefore, accurate acquisition of the convective heat transfer coefficient outside the micro heat exchange tube plays an important role in early-stage heat management analysis of practical engineering.
The method for measuring the convective heat transfer coefficient mainly comprises a steady state method and a transient state method, wherein the steady state method is simple in principle and simple and convenient to operate, but the experiment period is long; the transient method has short period and small error, but the experimental equipment is complex. Although the former adopts double working mediums to measure the convection heat transfer coefficient outside the tube, the multiple working mediums are based on the assumed condition that the formula of the heat transfer coefficient inside the tube is known; or assuming that the convection heat transfer coefficient in the tube is constant, solving by adopting a Wilson graphical method, and the method often has the problems of unreal assumed conditions and large test error.
The present invention has been made in view of this situation.
Disclosure of Invention
The invention aims to overcome the defects of the prior art and provides an improved method for measuring the heat transfer coefficient of the convection outside a micro-tube based on double working media. In order to solve the technical problems, the invention adopts the technical scheme that:
an improved method for measuring the convection heat transfer coefficient outside a micro-pipe based on double working media comprises the following steps:
Step 3, calculating physical property parameters of working media at the inner side of the tube according to the qualitative parameters in the tube;
Step 5, calculating the flow velocity V in the pipe medium And Reynolds number Re medium ；
Step 6, calculating the convective heat transfer coefficient h in the tube according to Dittus-Boelter empirical relation in ；
Step 7, calculating the logarithmic heat transfer temperature difference delta T according to the inlet and outlet temperatures of the air and the working medium lm ；
Step 8, obtaining the air heat exchange quantity Q in the heat balance according to the energy balance 2 Heat exchange quantity Q of working medium in tube 1 The heat exchange quantity Q;
step 10, calculating and obtaining the convection heat transfer coefficient h of the air side outside the pipe according to the heat transfer balance equation inside and outside the pipe out ：
Further, the physical parameters of the working medium in the pipe in the step 1 comprise the flow rate of the working medium
Working medium inlet temperature T in,medium Working medium outlet temperature T out,medium Physical parameters of air outside the pipe including air flow
Average air inlet temperature T in,air Average temperature T of air outlet out,air The physical parameters of the convection heat exchange environment comprise the inner and outer diameters D and D of the micro-fine tube bundle tubes and the number N of transverse tube rows T Number of rows of water inlet pipes N I Number of longitudinal tube rows N L Height H of heat exchange tube and transverse tube spacing s 1 And longitudinal tube spacing s 2 。
Further, the inside qualitative parameter T of the tube in the step 2 fmedium 、P fmedium Is of the formula
Wherein, T f,medium Denotes the qualitative temperature, P f,medium The qualitative pressure is indicated.
Further, the physical property parameter formula in the step 3 is
[ρ medium ,λ medium ,η medium ,C p,medium ,Pr medium ]＝f(T f,medium ,p f,medium )
Where ρ is medium Denotes density, λ medium Denotes the thermal conductivity, η medium Denotes dynamic viscosity, C pmedium Denotes specific heat at constant pressure, pr medium Representing the prandtl number.
Further, the calculation formula of the internal flow area and the external heat exchange area in the step 4 is
Further, the formula of the flow velocity and Reynolds number in the pipe in the step 5 is
Further, the heat convection coefficient h in the tubes in the step 6 in Is calculated by the formula
Further, the logarithmic heat transfer temperature difference calculation formula in the step 7 is
Further, the calculation formula of the heat exchange amount in the heat balance is
Further, the calculation formula for calculating the total heat transfer coefficient is
Wherein
Is a heat transfer coefficient correction factor.
After the technical scheme is adopted, compared with the prior art, the invention has the following beneficial effects.
The invention is based on that the measurement of the convective heat transfer coefficient is converted into the solution of the thermal resistance under the condition that two working media reach the thermal balance, and belongs to a steady state method with simple principle and simple and convenient operation. The invention has simple test equipment, convenient operation and small error of the experimental result; the method has high feasibility in theory and practice, and provides a guidance method for subsequent measurement of the external convection heat transfer coefficient of the micro-tube bundle.
The following describes embodiments of the present invention in further detail with reference to the accompanying drawings.
Drawings
The accompanying drawings, which are included to provide a further understanding of the invention and are incorporated in and constitute a part of this application, illustrate embodiment(s) of the invention and together with the description serve to explain the invention without limiting the invention to its proper form. It is obvious that the drawings in the following description are only some embodiments, and that for a person skilled in the art, other drawings can be derived from them without inventive effort. In the drawings:
FIG. 1 is a schematic representation of a three-dimensional structure model of the present invention;
FIG. 2 is a schematic diagram of the distribution of the direct measurement physical quantity of the present invention;
FIG. 3 is a schematic diagram of the computational error propagation process of the present invention.
It should be noted that the drawings and the description are not intended to limit the scope of the inventive concept in any way, but to illustrate it by a person skilled in the art with reference to specific embodiments.
Detailed Description
In order to make the objects, technical solutions and advantages of the embodiments of the present invention clearer, the technical solutions in the embodiments will be clearly and completely described below with reference to the drawings in the embodiments of the present invention, and the following embodiments are used for illustrating the present invention and are not intended to limit the scope of the present invention.
In the description of the present invention, it should be noted that the terms "upper", "lower", "front", "rear", "left", "right", "vertical", "inner", "outer", etc., indicate orientations or positional relationships based on the orientations or positional relationships shown in the drawings, and are only for convenience of description and simplicity of description, but do not indicate or imply that the device or element being referred to must have a particular orientation, be constructed and operated in a particular orientation, and thus, should not be construed as limiting the present invention.
In the description of the present invention, it should be noted that, unless otherwise explicitly specified or limited, the terms "mounted," "connected," and "connected" are to be construed broadly, e.g., as meaning either a fixed connection, a removable connection, or an integral connection; can be mechanically or electrically connected; may be directly connected or indirectly connected through an intermediate. The specific meanings of the above terms in the present invention can be understood in specific cases to those skilled in the art.
As shown in figures 1 to 3, the improved method for measuring the heat transfer coefficient of the external convection of the micro tube based on the double working mediums comprises the steps that as shown in a model of figure 1, the working medium in the tube heats the air outside the tube, when the working medium in the tube and the air outside the tube reach heat balance, the thermal parameters of the working medium inside and outside the micro tube bundle are measured, and the total thermal resistance of the whole heat transfer process is determined by combining an energy balance equation and a heat transfer equation, wherein a theoretical formula of the thermal conductivity and thermal resistance is calculated, a formula of the thermal resistance in the tube is calculated by an empirical formula, and the thermal resistance outside the tube is a substitute term. The method comprises the following steps:
pipe inner workingsMass side: flow of working medium
Working medium inlet temperature T in,medium Temperature T at working medium outlet out,medium ；
Air side outside the pipe: air flow rate
Average air inlet temperature T in,air Average temperature T of air outlet out,air ；
Measuring the inner diameter D and the outer diameter D of the micro-fine tube bundle tube and the number N of the transverse tube rows T Number of rows of water inlet pipes N I Number of longitudinal tube rows N L Height H of heat exchange tube and transverse tube spacing s 1 And longitudinal tube spacing s 2 As shown in fig. 2.
Wherein, T f,medium Denotes the qualitative temperature, P f,medium Indicating the qualitative pressure, where the subscript f can be replaced by in or out, indicating the qualitative temperature in or out, the qualitative pressure in or out, respectively.
Step 3, calculating the physical property parameters of the working medium at the inner side of the tube according to the qualitative parameters in the tube, wherein the formula is as follows:
[ρ medium ,λ medium ,η medium ,C p,medium ,Pr medium ]＝f(T f,medium ,p f,medium )
where ρ is medium Denotes density, λ medium Denotes the thermal conductivity, η medium Denotes dynamic viscosity, C pmedium Denotes specific heat at constant pressure, pr medium Representing the prandtl number.
step 5, calculating the flow velocity V in the pipe medium And Reynolds number Re medium ：
Step 6, calculating the convective heat transfer coefficient h in the tube according to the Dittus-Boelter empirical relation in ：
Step 7, calculating the logarithmic heat transfer temperature difference Delta T according to the inlet and outlet temperatures of the air and the working medium lm ；＝
Step 8, obtaining the air heat exchange quantity Q in the heat balance according to the energy balance 2 Heat exchange quantity Q of working medium in tube 1 And the heat exchange quantity Q is calculated according to the following formula:
wherein, C pmedium Indicating specific heat at constant pressure of working medium, C pair The specific heat at constant pressure of the heat exchange air is shown.
wherein
A heat transfer coefficient correction factor;
step 10, calculating and obtaining the convection heat transfer coefficient h of the air side outside the pipe according to the heat transfer balance equation inside and outside the pipe out ：
When the working medium inside and outside the pipe reaches thermal equilibrium, the heat convection coefficient h outside the pipe is calculated by the formula out The maximum error of (2) is 6.5%.
It can be seen from the above formula that the accuracy of the measurement of the convective heat transfer coefficient outside the tube is directly dependent on the accuracy of the measurement of the physical quantity directly, as well as the accuracy of the convective heat transfer coefficient inside the tube and the total convective heat transfer coefficient. The letter meanings indicated in the above formulae are well known to those skilled in the art. Under the existing technical level, the temperature, pressure and flow measurement of air and water have higher precision, and what needs to be analyzed in an important way is the error caused by the precision of the empirical relation of convection heat transfer in the pipe.
According to the error transmission process in the attached figure 3, errors of all parts are calculated:
(1) Measurement error
Er(T)、
Er (D) and Er (D), wherein the errors are defined by errors of a measuring instrument;
(2) Solving the error of Q according to a calculation formula of the heat exchange quantity Q:
(3) Solving the error of the total heat transfer coefficient K according to a calculation formula:
(4) According to engineering experience, the maximum deviation of the Dittus-Boelter empirical relationship from the true value is 30%, whereby h in The error of (2) is 30%;
(5) According to the heat convection coefficient h outside the tube out Is calculated by the calculation formula of (c) out Error of (2):
example one
As shown in fig. 1 to fig. 3, the present embodiment describes an improved method for measuring a heat transfer coefficient of a micro-tube external convection based on a bi-working medium. In order to verify the feasibility of the method, various physical quantities and errors under different working conditions of the external heat exchange of the small-scale tube bundle are calculated and analyzed, and the maximum error is selected to calculate the maximum deviation of the external convection heat exchange coefficient.
According to the common Reynolds number range of the external heat exchange of the micro-tube bundle and the operability of the measurement method, the following basic parameter conditions are given, and the measurement error of the convective heat exchange coefficient at the air side and the feasibility of the scheme are calculated and analyzed under the basic test conditions.
In the scheme, the working medium in the pipe is water, the outside of the pipe is air, the normal pressure state is normal, and the concrete data inside and outside the pipe are as follows:
the working medium side in the tube: flow of working medium
Working medium inlet temperature 90 deg.C, working medium outlet temperature T out,medium ；
Air side outside the pipe: air flow rate
Average temperature of air inlet 20 deg.C and average temperature of air outlet T out,air ；
Measuring the inner diameter D =1.1mm, the outer diameter D =1.5mm and the number of transverse tube rows N of the micro-fine tube bundle T Number of rows of water inlet pipes N =10 I Number of longitudinal tube rows N L The height H =150mm of the heat exchange tube;
the Reynolds number in the tube is 5000, and the Reynolds number outside the tube is considered to be 1000 and 4000.
Number of longitudinal rows considering 9 cases of 2, 3, 4, 6, 8, 12, 16, 20, the rows are arranged in an in-line manner, with each column making a single serpentine. Different numbers of lines will have different results and the calculations given below are the most probable of all the calculations.
According to the calculation formula of each heat exchange physical quantity in the invention content, each parameter value under each working condition is obtained through matlab iteration solution, and each error is calculated according to the error formula and the error transfer process.
And (3) calculating the result: in all the working conditions, the air conditioner is in a closed state,
water temperature reduction temperature: delta T water =4.7K-41.7K, maximum error: maxEr (. DELTA.T) water )＝9.7％；
Air temperature rise temperature: delta T air =5.7K-47.6K, maximum error: maxEr (. DELTA.T) air )＝4.9％；
Logarithmic mean temperature difference: delta T lm =33.8K-62.6K, maximum error: maxEr (. DELTA.T) lm )＝0.9％。
Calculating the maximum error maxEr (Q) =5.5% of the heat exchange quantity Q and the maximum error maxEr (K) =5.8% of the total heat transfer coefficient K according to the error transfer, so that the convection heat transfer coefficient h outside the tube is obtained out Maximum error of (h) maxEr out ) =6.5%. Passing by mistakeThe difference analysis proves that the maximum error of the method is within an acceptable range, so that the error of the method for measuring the heat exchange coefficient outside the micro-tube based on the double working mediums meets the engineering calculation requirement.
The test equipment comprises a heat exchange cavity, a plurality of micro-tubes uniformly distributed, a circulating water system and an air supply system, wherein the air supply system is connected with the heat exchange cavity, so that air flow is formed in the heat exchange cavity, and the micro-tubes are arranged in a direction vertical to the flow direction of the air flow. Two ends of the circulating water system are respectively communicated with two ends of the microtube, so that circulating water is introduced into the microtube. The flow direction of water in the microtubes is perpendicular to the flow direction of the air flow, and the air flow is in contact with the outer side walls of the microtubes and flows between the microtubes, so that the water in the microtubes can be subjected to sufficient heat exchange. A circulating water system can be formed by a common water pump, a water tank and a heater on the market; air is outside the pipe, and equipment such as a common centrifugal fan in the market and the like can be selected to meet the air supply condition of the test; in addition, the temperature, pressure and flow measurement required by the method are all in the application range of common measurement equipment, and high-precision thermal resistors, pressure sensors and thermal flow meters can be conveniently selected as the measurement equipment to finish direct measurement of physical quantity. The equipment and process for measurement are all the prior art, and are only used for measurement application, and no technical improvement is needed, so that detailed structures and working principles are not described herein.
In conclusion, the method has high feasibility in both theoretical and practical convenience, and provides a guidance method for subsequent measurement of the external convective heat transfer coefficient of the micro-tube bundle.
Although the present invention has been described with reference to a preferred embodiment, it should be understood that various changes, substitutions and alterations can be made herein without departing from the spirit and scope of the invention as defined by the appended claims.
Claims (10)
1. An improved method for measuring the heat transfer coefficient of the external convection of a micro-pipe based on double working media is characterized by comprising the following steps:
step 1, measuring physical parameters of working media in a pipe and air outside the pipe, and determining the physical parameters of a convective heat transfer environment;
step 2, calculating a qualitative parameter T at the inner side of the tube according to the physical parameters of the working medium in the tube f 、P f ；
Step 3, calculating physical property parameters of working media at the inner side of the tube according to qualitative parameters in the tube;
step 4, calculating the flow area A in the pipe fmedium And total heat transfer area A outside the tube hair ；
Step 5, calculating the flow velocity V in the pipe medium And Reynolds number Re medium ；
Step 6, calculating the convective heat transfer coefficient h in the tube according to Dittus-Boelter empirical relation in ；
Step 7, calculating the logarithmic heat transfer temperature difference delta T according to the inlet and outlet temperatures of the air and the working medium lm ；
Step 8, obtaining the air heat exchange quantity Q in the heat balance according to the energy balance 2 And heat exchange quantity Q of working medium in pipe 1 And the heat exchange quantity Q;
step 9, calculating a total heat transfer coefficient K based on the total heat transfer area outside the pipe;
step 10, according to the heat transfer balance equation inside and outside the tube, calculating to obtain the heat convection coefficient h of the air side outside the tube out ：
2. The improved method for measuring the heat transfer coefficient of the external convection of the micro-tube based on the bi-working medium as claimed in claim 1, wherein: the physical parameters of the working medium in the pipe in the step 1 comprise the flow of the working medium
Working medium inlet temperature T in,medium Working medium outlet temperature T out,medium Physical parameters of air outside the pipe including air flow
Average air inlet temperature T in,air Average temperature T of air outlet out,air The physical parameters of the convection heat exchange environment comprise the inner and outer diameters D and D of the micro-fine tube bundle tubes and the number N of transverse tube rows T Number of rows of water inlet pipes N I Number of longitudinal tube rows N L Height H of heat exchange tube and transverse tube spacing s 1 And longitudinal tube spacing s 2 。
3. The improved method for measuring the heat transfer coefficient of the double-working-medium-based micro-tube external convection heat exchange as claimed in claim 1, is characterized in that: the qualitative parameter T of the inner side of the pipe in the step 2 f,medium 、P f,medium Is of the formula
Wherein, T f,medium Denotes the qualitative temperature, P f,medium The qualitative pressure is indicated.
4. The improved method for measuring the heat transfer coefficient of the double-working-medium-based micro-tube external convection heat exchange as claimed in claim 1, is characterized in that: the physical property parameter formula in the step 3 is
[ρ medium ,λ medium ,η medium ,C p,medium ,Pr medium ]＝f(T f,medium ,p f,medium )
Where ρ is medium Denotes density, λ medium Denotes the thermal conductivity, η medium Denotes dynamic viscosity, C pmedium Denotes specific heat at constant pressure, pr medium Representing the prandtl number。
5. The improved method for measuring the heat transfer coefficient of the external convection of the micro-tube based on the bi-working medium as claimed in claim 1, wherein: the calculation formula of the internal flow area and the external heat exchange area in the step 4 is
6. The improved method for measuring the heat transfer coefficient of the double-working-medium-based micro-tube external convection heat exchange as claimed in claim 1, is characterized in that: the flow velocity V in the tube in the step 5 medium Reynolds number Re medium Is of the formula
7. The improved method for measuring the heat transfer coefficient of the external convection of the micro-tube based on the bi-working medium as claimed in claim 1, wherein: in the step 6 of the inside of the pipe Nu of Nu medium And convective heat transfer coefficient h in Is calculated by the formula
8. The improved method for measuring the heat transfer coefficient of the double-working-medium-based micro-tube external convection heat exchange as claimed in claim 1, is characterized in that: the logarithmic heat transfer temperature difference Delta T in the step 7 lm Is calculated by the formula
9. The improved method for measuring the heat transfer coefficient of the double-working-medium-based micro-tube external convection heat exchange as claimed in claim 1, is characterized in that: the calculation formula of the heat exchange quantity Q in the heat balance is
10. The improved method for measuring the heat transfer coefficient of the external convection of the micro-tube based on the bi-working medium as claimed in claim 1, wherein: the calculation formula for calculating the total heat transfer coefficient K is
Wherein
Is a heat transfer coefficient correction factor.
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