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The paper presents the experimentally investigated operation modes of a flat loop heat pipe (LHP). The LHP is an efficient heat transfer device operating on the principle of evaporation-condensation cycle and successfully applied in space technology, including cooling heat-stressed components of electronic devices and computer equipment.We have experimentally studied how design parameters of the vapor line and its coolant flow influence on the LHP operation mode and also have determined the causes for emerging oscillatory mode of the LHP operation at low heat load. The paper depicts the experimentally measured temperatures in the LHP characteristic points and the photographs of the coolant flow in the vapor line.Based on the experimental data, we have drawn the following conclusions:A vapor-liquid coolant flow in the vapor line in the range of the heat loads under consideration has been detected. There is no superheating vapour observed.The flow regime of the vapor-liquid mixture depends on both the heat load and the vapor pipe diameter. The decrease in the internal diameter of the investigated vapor line section from 7 mm to 4 mm led to the increase of its vapor content and to the decrease of the heating surface temperature when the heat loads were above 80 W. For example, the temperature of the heating surface T1 decreased from 109.5 °С to 100 °С at a heat load of 110 W. Reducing the heat load from 80 W to 60 W leads to a change in the flow regime of the vapor-water mixture from the annular to the slug regime. Found that at low heat loads (up to 40 W), there is no LHP loop operation observed. Periodic fluctuations in the water level in the vapor line are detected. The LHP operates in thermo-syphon mode. For these heat loads, the influence of the vapor line diameter on the thermal state of the LHP is not observed.Found that at low heat loads the LHP operation mode depends only on the flow regime of the coolant in the vapor line. With the annular regime of the coolant flow in the vapor line, a stationary mode of operation of the LHP is observed. When changing the flow regime of the coolant from the annular to the slug, the LHP operation mode is changed from stationary to oscillatory.