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通常用来测试换热面放热性能的常规方法是先在稳定态下测量其换热面和流体的平均温度以及换热率,然后按照牛顿冷却定律将换热率除以流体和换热面的平均温度差来计算其平均放热系数h。显然,用这种方法求得的放热系数h的准确度主要取决于换热面和流体的温度测量误差;而对高效,紧凑换热面来说,温度测量误差的影响尤为严重。可是恰恰对高效,紧凑换热面来说,平均壁温的测量是十分困难的,即使勉力而为,也难以获得准确结果。这是因为为了测量换热面各处温度而装在翅片面(很薄而且相邻两翅片间的距离又只有1.5mm左右)上的测温元件本身将显著影响试件芯子内的温度分布和速度分布。这就是目前紧凑式换热面的放热性能的资料大多是用各种不需测定壁温的方法来求得的原因。这种不需测定壁温来确定换热面性能的方法巳发展有很多种,它们可区分为两大类:稳定态法和瞬变态法;而每一类又包含很多种。它们的理论根据,特点和适用范围都互不相同。因此,要获得准确结果,就必须根据试件的特点和测量范围,来选用合适的方法并仔细地保证规定的试验条件。本文将对各种不需测定壁温来确定放热性能的方法作一综合评价,给出它们的优缺点和适用范围,以便正确使用这些方法来确定各种紧凑式换热面的放热性能和合理评价用各种不同方法所求得的实验结果。
Commonly used to test the exothermic surface heat transfer performance of the conventional method is first measured in the steady state of the heat transfer surface and the average fluid temperature and heat transfer rate, and then according to Newton’s law of heat transfer rate will be divided by the fluid and heat transfer surface The average temperature difference to calculate the average heat release coefficient h. Obviously, the accuracy of the heat release coefficient h obtained by this method mainly depends on the temperature measurement errors of the heat exchange surface and the fluid, and the temperature measurement error is particularly serious for the efficient and compact heat exchange surface. But just for efficient, compact heat transfer surface, the average wall temperature measurement is very difficult, even if managed to, it is difficult to obtain accurate results. This is because the temperature sensing element itself mounted on the fin surface (thin and with a distance of only about 1.5 mm between adjacent two fins) in order to measure the temperature across the heat transfer surface will significantly affect the temperature in the core Distribution and speed distribution. This is the reason why most of the information on the exothermic performance of compact heat exchange surfaces is obtained by various methods without measuring the wall temperature. There are a number of ways in which this wall-temperature determination can be made to determine the performance of the heat transfer surfaces. There are a number of different ways in which they can be differentiated into two broad categories: steady-state and transient; and each includes many more. Their theoretical basis, characteristics and scope of application are different from each other. Therefore, to obtain accurate results, it is necessary to select the appropriate method and carefully ensure the specified test conditions according to the characteristics of the test piece and the measuring range. In this paper, we will give a comprehensive evaluation of various methods of determining the exothermic properties without determining the wall temperature, and give their advantages and disadvantages as well as their applicability so that these methods can be used correctly to determine the exothermic performance of various compact heat exchange surfaces And a reasonable evaluation of the results obtained using a variety of different methods.