Heated length

Correct hj by multiplying by the tube size and heated length factors accompanying Figure 10-49. 

Heated Length Correction Factors, Streamline Flow Tube Size Correction Factors for Streamline Flow ... 

D4. Doroshtchuk, V. E., and Frid, F. P., On the problem of the influence of inlet throttling and of the heated length on the critical heat flux for round tubes, Teploenerg. (Sept. 1959). 

With the heated length for the 0.8 mm diameter mbe enlarged from 20 to 70 mm, the drop in the CHF value is chf = 400 kW/m. It confirms that the heated length, besides the mass velocity, is one of the most important parameters in the design of heat sinks. 

The micro-channels utilized in engineering systems are frequently connected with inlet and outlet manifolds. In this case the thermal boundary condition at the inlet and outlet of the tube is not adiabatic. Heat transfer in a micro-tube under these conditions was studied by Hetsroni et al. (2004). They measured heat transfer to water flowing in a pipe of inner diameter 1.07 mm, outer diameter 1.5 mm, and 0.600 m in length, as shown in Fig. 4.2b. The pipe was divided into two sections. The development section of Lj = 0.245 m was used to obtain fully developed flow and thermal fields. The test section proper, of heating length Lh = 0.335 m, was used for collecting the experimental data. 

Author Number of channels in the test section Heating length Lii [mm] Hydraulic diameter [pm] Dimensionless length Lh/4h Reynolds number Re... 

Experiments of propane pyrolysis were carried out using a thin tubular CVD reactor as shown in Fig. 1 [4]. The inner diameter and heating length of the tube were 4.8 mm and 30 cm, respectively. Temperature was around 1000°C. Propane pressure was 0.1-6.7 kPa. Total pressure was 6.7 kPa. Helium was used as carrier gas. The product gas was analyzed by gas chromatography and the carbon deposition rate was calculated from the film thickness measured by electron microscopy. The effects of the residence time and the temperature... 

AHail is enthalpy gained along heated length, kJ/kg HL and HG are enthalpy of water and steam, respectively, kJ/kg pL and pG are density of water and steam, respectively, kg/m3... 

Hfg = latent heat of vaporization, J/g L = overall heated length, cm Lb = boiling length, cm n = number of rods, dimensionless P = perimeter, cm p = pressure, kg cm2 WB = power over the boiling length, kW = overall power at CHF, kW Xcr = quality (weight percentage) at CHF, dimensionless... 

By cutting out a section of heated length of a Freon loop at the inlet and restoring the original flow rate, Crowley et al. (1967) found that the reduction of the heated length increased the flow stability in forced circulation with a constant power density. A similar effect was found in a natural-circulation loop (Mathisen, 1967). Crowley et al. (1967) further noticed that the change of heated length did not affect the period of oscillation, since the flow rate was kept constant. 

Heated length of reformer tubes 13.72 m Shape Raschig18 rings... 

Feed/effluent heat exchangers are used in many industrial processes to warm up the fluid before the reactor and to cool it down after treatment at high temperature. The conventional design of such heat exchangers is based on shell-and-tube units. But to increase the thermal effectiveness of the heat exchangers, the required heat length becomes very important, and high pressure drop will occur. 

Unheated length at bottom of die tube Total length Heated length/diameter Water cooling... 

L = heated length of straight tube, ft w = weight rate of flow per tube, lb/h... 

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